diff --git a/.eslintignore b/.eslintignore
index b1ed7dff30e9..b4e761b0a491 100644
--- a/.eslintignore
+++ b/.eslintignore
@@ -169,7 +169,7 @@ remote/cdp/test/browser/chrome-remote-interface.js
 remote/marionette/atom.js
 
 # This file explicitly has a syntax error and cannot be parsed by eslint.
-remote/shared/messagehandler/test/browser/resources/modules/root/invalid.jsm
+remote/shared/messagehandler/test/browser/resources/modules/root/invalid.sys.mjs
 
 # services/ exclusions
 
diff --git a/browser/base/content/test/performance/browser_startup_content.js b/browser/base/content/test/performance/browser_startup_content.js
index db92a2ce6d82..2cbdee4e2733 100644
--- a/browser/base/content/test/performance/browser_startup_content.js
+++ b/browser/base/content/test/performance/browser_startup_content.js
@@ -83,7 +83,7 @@ const intermittently_loaded_scripts = {
 
     // Test related
     "chrome://remote/content/marionette/actors/MarionetteEventsChild.jsm",
-    "chrome://remote/content/shared/Log.jsm",
+    "chrome://remote/content/shared/Log.sys.mjs",
     "resource://testing-common/BrowserTestUtilsChild.jsm",
     "resource://testing-common/ContentEventListenerChild.jsm",
     "resource://specialpowers/AppTestDelegateChild.jsm",
diff --git a/browser/base/content/test/performance/browser_startup_content_subframe.js b/browser/base/content/test/performance/browser_startup_content_subframe.js
index b046cc07406b..090a90ee76fe 100644
--- a/browser/base/content/test/performance/browser_startup_content_subframe.js
+++ b/browser/base/content/test/performance/browser_startup_content_subframe.js
@@ -72,7 +72,7 @@ const intermittently_loaded_scripts = {
 
     // Test related
     "chrome://remote/content/marionette/actors/MarionetteEventsChild.jsm",
-    "chrome://remote/content/shared/Log.jsm",
+    "chrome://remote/content/shared/Log.sys.mjs",
     "resource://testing-common/BrowserTestUtilsChild.jsm",
     "resource://testing-common/ContentEventListenerChild.jsm",
     "resource://specialpowers/SpecialPowersChild.jsm",
diff --git a/remote/cdp/CDP.jsm b/remote/cdp/CDP.jsm
index 046260911df1..1e3483d46b8c 100644
--- a/remote/cdp/CDP.jsm
+++ b/remote/cdp/CDP.jsm
@@ -12,11 +12,14 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  RecommendedPreferences:
+    "chrome://remote/content/shared/RecommendedPreferences.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   JSONHandler: "chrome://remote/content/cdp/JSONHandler.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
-  RecommendedPreferences:
-    "chrome://remote/content/shared/RecommendedPreferences.jsm",
   TargetList: "chrome://remote/content/cdp/targets/TargetList.jsm",
 });
 
diff --git a/remote/cdp/CDPConnection.jsm b/remote/cdp/CDPConnection.jsm
index 12805d830b70..c9d8d099301e 100644
--- a/remote/cdp/CDPConnection.jsm
+++ b/remote/cdp/CDPConnection.jsm
@@ -10,14 +10,17 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
   "resource://gre/modules/XPCOMUtils.sys.mjs"
 );
 
-const { WebSocketConnection } = ChromeUtils.import(
-  "chrome://remote/content/shared/WebSocketConnection.jsm"
+const { WebSocketConnection } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/WebSocketConnection.sys.mjs"
 );
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  Log: "chrome://remote/content/shared/Log.jsm",
   UnknownMethodError: "chrome://remote/content/cdp/Error.jsm",
 });
 
diff --git a/remote/cdp/Error.jsm b/remote/cdp/Error.jsm
index 66759734cd20..74bfef125ed7 100644
--- a/remote/cdp/Error.jsm
+++ b/remote/cdp/Error.jsm
@@ -17,8 +17,8 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  Log: "chrome://remote/content/shared/Log.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/cdp/JSONHandler.jsm b/remote/cdp/JSONHandler.jsm
index 33b2fdd7c398..bcf175940429 100644
--- a/remote/cdp/JSONHandler.jsm
+++ b/remote/cdp/JSONHandler.jsm
@@ -12,10 +12,13 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   HTTP_404: "chrome://remote/content/server/HTTPD.jsm",
   HTTP_505: "chrome://remote/content/server/HTTPD.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
   Protocol: "chrome://remote/content/cdp/Protocol.jsm",
   RemoteAgentError: "chrome://remote/content/cdp/Error.jsm",
 });
diff --git a/remote/cdp/domains/content/Emulation.jsm b/remote/cdp/domains/content/Emulation.jsm
index 104cb8e4410c..d6f5af1b88bb 100644
--- a/remote/cdp/domains/content/Emulation.jsm
+++ b/remote/cdp/domains/content/Emulation.jsm
@@ -6,18 +6,14 @@
 
 var EXPORTED_SYMBOLS = ["Emulation"];
 
-var { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
 const { ContentProcessDomain } = ChromeUtils.import(
   "chrome://remote/content/cdp/domains/ContentProcessDomain.jsm"
 );
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  AnimationFramePromise: "chrome://remote/content/shared/Sync.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  AnimationFramePromise: "chrome://remote/content/shared/Sync.sys.mjs",
 });
 
 class Emulation extends ContentProcessDomain {
diff --git a/remote/cdp/domains/content/Runtime.jsm b/remote/cdp/domains/content/Runtime.jsm
index 542e5db0085f..63b93afa6e37 100644
--- a/remote/cdp/domains/content/Runtime.jsm
+++ b/remote/cdp/domains/content/Runtime.jsm
@@ -19,8 +19,11 @@ const { ContentProcessDomain } = ChromeUtils.import(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  executeSoon: "chrome://remote/content/shared/Sync.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  executeSoon: "chrome://remote/content/shared/Sync.jsm",
   ExecutionContext:
     "chrome://remote/content/cdp/domains/content/runtime/ExecutionContext.jsm",
 });
diff --git a/remote/cdp/domains/parent/Page.jsm b/remote/cdp/domains/parent/Page.jsm
index 34b023f27431..c96d87f7cc28 100644
--- a/remote/cdp/domains/parent/Page.jsm
+++ b/remote/cdp/domains/parent/Page.jsm
@@ -16,6 +16,12 @@ const { Domain } = ChromeUtils.import(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  PollPromise: "chrome://remote/content/shared/Sync.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+  windowManager: "chrome://remote/content/shared/WindowManager.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   clearInterval: "resource://gre/modules/Timer.jsm",
   OS: "resource://gre/modules/osfile.jsm",
@@ -24,11 +30,8 @@ XPCOMUtils.defineLazyModuleGetters(lazy, {
 
   DialogHandler:
     "chrome://remote/content/cdp/domains/parent/page/DialogHandler.jsm",
-  PollPromise: "chrome://remote/content/shared/Sync.jsm",
   streamRegistry: "chrome://remote/content/cdp/domains/parent/IO.jsm",
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
   UnsupportedError: "chrome://remote/content/cdp/Error.jsm",
-  windowManager: "chrome://remote/content/shared/WindowManager.jsm",
 });
 
 const MAX_CANVAS_DIMENSION = 32767;
diff --git a/remote/cdp/domains/parent/Target.jsm b/remote/cdp/domains/parent/Target.jsm
index b6c1e83f8066..f8369c0e348b 100644
--- a/remote/cdp/domains/parent/Target.jsm
+++ b/remote/cdp/domains/parent/Target.jsm
@@ -16,15 +16,18 @@ const { Domain } = ChromeUtils.import(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+  windowManager: "chrome://remote/content/shared/WindowManager.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   ContextualIdentityService:
     "resource://gre/modules/ContextualIdentityService.jsm",
 
   MainProcessTarget:
     "chrome://remote/content/cdp/targets/MainProcessTarget.jsm",
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
   TabSession: "chrome://remote/content/cdp/sessions/TabSession.jsm",
-  windowManager: "chrome://remote/content/shared/WindowManager.jsm",
 });
 
 let browserContextIds = 1;
diff --git a/remote/cdp/observers/ContextObserver.jsm b/remote/cdp/observers/ContextObserver.jsm
index 1deabd27e0db..4da62652f832 100644
--- a/remote/cdp/observers/ContextObserver.jsm
+++ b/remote/cdp/observers/ContextObserver.jsm
@@ -31,10 +31,12 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  executeSoon: "chrome://remote/content/shared/Sync.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   EventEmitter: "resource://gre/modules/EventEmitter.jsm",
-
-  executeSoon: "chrome://remote/content/shared/Sync.jsm",
 });
 
 class ContextObserver {
diff --git a/remote/cdp/observers/TargetObserver.jsm b/remote/cdp/observers/TargetObserver.jsm
index 6e5bdc69a214..30f1a3f92df7 100644
--- a/remote/cdp/observers/TargetObserver.jsm
+++ b/remote/cdp/observers/TargetObserver.jsm
@@ -12,10 +12,12 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  EventPromise: "chrome://remote/content/shared/Sync.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   EventEmitter: "resource://gre/modules/EventEmitter.jsm",
-
-  EventPromise: "chrome://remote/content/shared/Sync.jsm",
 });
 
 // TODO(ato):
diff --git a/remote/cdp/targets/TabTarget.jsm b/remote/cdp/targets/TabTarget.jsm
index 58e43a7cc8c5..fcaecc1d9133 100644
--- a/remote/cdp/targets/TabTarget.jsm
+++ b/remote/cdp/targets/TabTarget.jsm
@@ -16,9 +16,12 @@ const { Target } = ChromeUtils.import(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   RemoteAgent: "chrome://remote/content/components/RemoteAgent.jsm",
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
   TabSession: "chrome://remote/content/cdp/sessions/TabSession.jsm",
 });
 
diff --git a/remote/cdp/targets/TargetList.jsm b/remote/cdp/targets/TargetList.jsm
index e8faf1f77eb9..f21000b8acad 100644
--- a/remote/cdp/targets/TargetList.jsm
+++ b/remote/cdp/targets/TargetList.jsm
@@ -12,11 +12,15 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   EventEmitter: "resource://gre/modules/EventEmitter.jsm",
+
   MainProcessTarget:
     "chrome://remote/content/cdp/targets/MainProcessTarget.jsm",
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
   TabObserver: "chrome://remote/content/cdp/observers/TargetObserver.jsm",
   TabTarget: "chrome://remote/content/cdp/targets/TabTarget.jsm",
 });
diff --git a/remote/cdp/test/browser/head.js b/remote/cdp/test/browser/head.js
index cadd6e2f2f89..6c6e99210b36 100644
--- a/remote/cdp/test/browser/head.js
+++ b/remote/cdp/test/browser/head.js
@@ -11,8 +11,8 @@ const { RemoteAgent } = ChromeUtils.import(
 const { RemoteAgentError } = ChromeUtils.import(
   "chrome://remote/content/cdp/Error.jsm"
 );
-const { TabManager } = ChromeUtils.import(
-  "chrome://remote/content/shared/TabManager.jsm"
+const { TabManager } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/TabManager.sys.mjs"
 );
 
 const TIMEOUT_MULTIPLIER = SpecialPowers.isDebugBuild ? 4 : 1;
diff --git a/remote/cdp/test/browser/input/head.js b/remote/cdp/test/browser/input/head.js
index 96540ccc5b46..fe085fc7b2ae 100644
--- a/remote/cdp/test/browser/input/head.js
+++ b/remote/cdp/test/browser/input/head.js
@@ -13,8 +13,8 @@ Services.scriptloader.loadSubScript(
 const { Input: I } = ChromeUtils.import(
   "chrome://remote/content/cdp/domains/parent/Input.jsm"
 );
-const { AppInfo } = ChromeUtils.import(
-  "chrome://remote/content/shared/AppInfo.jsm"
+const { AppInfo } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/AppInfo.sys.mjs"
 );
 
 const { alt, ctrl, meta, shift } = I.Modifier;
diff --git a/remote/cdp/test/browser/page/head.js b/remote/cdp/test/browser/page/head.js
index efea7b72bd77..ff25b83c5cf7 100644
--- a/remote/cdp/test/browser/page/head.js
+++ b/remote/cdp/test/browser/page/head.js
@@ -17,8 +17,8 @@ const {
   setTimeout,
 } = ChromeUtils.import("resource://gre/modules/Timer.jsm");
 
-const { PollPromise } = ChromeUtils.import(
-  "chrome://remote/content/shared/Sync.jsm"
+const { PollPromise } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/Sync.sys.mjs"
 );
 
 const BASE_ORIGIN = "https://example.com";
diff --git a/remote/components/Marionette.jsm b/remote/components/Marionette.jsm
index 2418a4e4672d..3cf3dcc39258 100644
--- a/remote/components/Marionette.jsm
+++ b/remote/components/Marionette.jsm
@@ -12,14 +12,18 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  Deferred: "chrome://remote/content/shared/Sync.jsm",
-  EnvironmentPrefs: "chrome://remote/content/marionette/prefs.js",
-  Log: "chrome://remote/content/shared/Log.jsm",
-  MarionettePrefs: "chrome://remote/content/marionette/prefs.js",
-  Preferences: "resource://gre/modules/Preferences.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  Deferred: "chrome://remote/content/shared/Sync.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
   RecommendedPreferences:
-    "chrome://remote/content/shared/RecommendedPreferences.jsm",
+    "chrome://remote/content/shared/RecommendedPreferences.sys.mjs",
+});
+
+XPCOMUtils.defineLazyModuleGetters(lazy, {
+  Preferences: "resource://gre/modules/Preferences.jsm",
+
+  EnvironmentPrefs: "chrome://remote/content/marionette/prefs.js",
+  MarionettePrefs: "chrome://remote/content/marionette/prefs.js",
   TCPListener: "chrome://remote/content/marionette/server.js",
 });
 
diff --git a/remote/components/RemoteAgent.jsm b/remote/components/RemoteAgent.jsm
index d5d72b24f5cb..713478d6481c 100644
--- a/remote/components/RemoteAgent.jsm
+++ b/remote/components/RemoteAgent.jsm
@@ -12,11 +12,14 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  Deferred: "chrome://remote/content/shared/Sync.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   CDP: "chrome://remote/content/cdp/CDP.jsm",
-  Deferred: "chrome://remote/content/shared/Sync.jsm",
   HttpServer: "chrome://remote/content/server/HTTPD.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
   WebDriverBiDi: "chrome://remote/content/webdriver-bidi/WebDriverBiDi.jsm",
 });
 
diff --git a/remote/jar.mn b/remote/jar.mn
index 9e3c6001f015..3cb2f202c9a0 100644
--- a/remote/jar.mn
+++ b/remote/jar.mn
@@ -13,48 +13,48 @@ remote.jar:
   content/server/WebSocketTransport.jsm (server/WebSocketTransport.jsm)
 
   # shared modules (all protocols)
-  content/shared/AppInfo.jsm (shared/AppInfo.jsm)
-  content/shared/Format.jsm (shared/Format.jsm)
-  content/shared/Log.jsm (shared/Log.jsm)
-  content/shared/MobileTabBrowser.jsm (shared/MobileTabBrowser.jsm)
-  content/shared/Navigate.jsm (shared/Navigate.jsm)
-  content/shared/PDF.jsm (shared/PDF.jsm)
-  content/shared/RecommendedPreferences.jsm (shared/RecommendedPreferences.jsm)
-  content/shared/RemoteError.jsm (shared/RemoteError.jsm)
-  content/shared/Stack.jsm (shared/Stack.jsm)
-  content/shared/Sync.jsm (shared/Sync.jsm)
-  content/shared/TabManager.jsm (shared/TabManager.jsm)
-  content/shared/WebSocketConnection.jsm (shared/WebSocketConnection.jsm)
-  content/shared/WindowManager.jsm (shared/WindowManager.jsm)
-  content/shared/listeners/BrowsingContextListener.jsm (shared/listeners/BrowsingContextListener.jsm)
-  content/shared/listeners/ConsoleAPIListener.jsm (shared/listeners/ConsoleAPIListener.jsm)
-  content/shared/listeners/ConsoleListener.jsm (shared/listeners/ConsoleListener.jsm)
-  content/shared/listeners/LoadListener.jsm (shared/listeners/LoadListener.jsm)
+  content/shared/AppInfo.sys.mjs (shared/AppInfo.sys.mjs)
+  content/shared/Format.sys.mjs (shared/Format.sys.mjs)
+  content/shared/Log.sys.mjs (shared/Log.sys.mjs)
+  content/shared/MobileTabBrowser.sys.mjs (shared/MobileTabBrowser.sys.mjs)
+  content/shared/Navigate.sys.mjs (shared/Navigate.sys.mjs)
+  content/shared/PDF.sys.mjs (shared/PDF.sys.mjs)
+  content/shared/RecommendedPreferences.sys.mjs (shared/RecommendedPreferences.sys.mjs)
+  content/shared/RemoteError.sys.mjs (shared/RemoteError.sys.mjs)
+  content/shared/Stack.sys.mjs (shared/Stack.sys.mjs)
+  content/shared/Sync.sys.mjs (shared/Sync.sys.mjs)
+  content/shared/TabManager.sys.mjs (shared/TabManager.sys.mjs)
+  content/shared/WebSocketConnection.sys.mjs (shared/WebSocketConnection.sys.mjs)
+  content/shared/WindowManager.sys.mjs (shared/WindowManager.sys.mjs)
+  content/shared/listeners/BrowsingContextListener.sys.mjs (shared/listeners/BrowsingContextListener.sys.mjs)
+  content/shared/listeners/ConsoleAPIListener.sys.mjs (shared/listeners/ConsoleAPIListener.sys.mjs)
+  content/shared/listeners/ConsoleListener.sys.mjs (shared/listeners/ConsoleListener.sys.mjs)
+  content/shared/listeners/LoadListener.sys.mjs (shared/listeners/LoadListener.sys.mjs)
 
   # shared modules (messagehandler architecture)
-  content/shared/messagehandler/Errors.jsm (shared/messagehandler/Errors.jsm)
-  content/shared/messagehandler/EventsDispatcher.jsm (shared/messagehandler/EventsDispatcher.jsm)
-  content/shared/messagehandler/MessageHandler.jsm (shared/messagehandler/MessageHandler.jsm)
-  content/shared/messagehandler/MessageHandlerRegistry.jsm (shared/messagehandler/MessageHandlerRegistry.jsm)
-  content/shared/messagehandler/Module.jsm (shared/messagehandler/Module.jsm)
-  content/shared/messagehandler/ModuleCache.jsm (shared/messagehandler/ModuleCache.jsm)
-  content/shared/messagehandler/RootMessageHandler.jsm (shared/messagehandler/RootMessageHandler.jsm)
-  content/shared/messagehandler/RootMessageHandlerRegistry.jsm (shared/messagehandler/RootMessageHandlerRegistry.jsm)
-  content/shared/messagehandler/WindowGlobalMessageHandler.jsm (shared/messagehandler/WindowGlobalMessageHandler.jsm)
-  content/shared/messagehandler/sessiondata/SessionData.jsm (shared/messagehandler/sessiondata/SessionData.jsm)
-  content/shared/messagehandler/sessiondata/SessionDataReader.jsm (shared/messagehandler/sessiondata/SessionDataReader.jsm)
-  content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.jsm (shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.jsm)
-  content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm (shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm)
-  content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm (shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm)
-  content/shared/messagehandler/transports/FrameContextUtils.jsm (shared/messagehandler/transports/FrameContextUtils.jsm)
-  content/shared/messagehandler/transports/FrameTransport.jsm (shared/messagehandler/transports/FrameTransport.jsm)
+  content/shared/messagehandler/Errors.sys.mjs (shared/messagehandler/Errors.sys.mjs)
+  content/shared/messagehandler/EventsDispatcher.sys.mjs (shared/messagehandler/EventsDispatcher.sys.mjs)
+  content/shared/messagehandler/MessageHandler.sys.mjs (shared/messagehandler/MessageHandler.sys.mjs)
+  content/shared/messagehandler/MessageHandlerRegistry.sys.mjs (shared/messagehandler/MessageHandlerRegistry.sys.mjs)
+  content/shared/messagehandler/Module.sys.mjs (shared/messagehandler/Module.sys.mjs)
+  content/shared/messagehandler/ModuleCache.sys.mjs (shared/messagehandler/ModuleCache.sys.mjs)
+  content/shared/messagehandler/RootMessageHandler.sys.mjs (shared/messagehandler/RootMessageHandler.sys.mjs)
+  content/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs (shared/messagehandler/RootMessageHandlerRegistry.sys.mjs)
+  content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs (shared/messagehandler/WindowGlobalMessageHandler.sys.mjs)
+  content/shared/messagehandler/sessiondata/SessionData.sys.mjs (shared/messagehandler/sessiondata/SessionData.sys.mjs)
+  content/shared/messagehandler/sessiondata/SessionDataReader.sys.mjs (shared/messagehandler/sessiondata/SessionDataReader.sys.mjs)
+  content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.sys.mjs (shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.sys.mjs)
+  content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.sys.mjs (shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.sys.mjs)
+  content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.sys.mjs (shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.sys.mjs)
+  content/shared/messagehandler/transports/FrameContextUtils.sys.mjs (shared/messagehandler/transports/FrameContextUtils.sys.mjs)
+  content/shared/messagehandler/transports/FrameTransport.sys.mjs (shared/messagehandler/transports/FrameTransport.sys.mjs)
 
   # shared modules (WebDriver HTTP / BiDi only)
-  content/shared/webdriver/Assert.jsm (shared/webdriver/Assert.jsm)
-  content/shared/webdriver/Capabilities.jsm (shared/webdriver/Capabilities.jsm)
-  content/shared/webdriver/Errors.jsm (shared/webdriver/Errors.jsm)
-  content/shared/webdriver/KeyData.jsm (shared/webdriver/KeyData.jsm)
-  content/shared/webdriver/Session.jsm (shared/webdriver/Session.jsm)
+  content/shared/webdriver/Assert.sys.mjs (shared/webdriver/Assert.sys.mjs)
+  content/shared/webdriver/Capabilities.sys.mjs (shared/webdriver/Capabilities.sys.mjs)
+  content/shared/webdriver/Errors.sys.mjs (shared/webdriver/Errors.sys.mjs)
+  content/shared/webdriver/KeyData.sys.mjs (shared/webdriver/KeyData.sys.mjs)
+  content/shared/webdriver/Session.sys.mjs (shared/webdriver/Session.sys.mjs)
 
   # imports from external folders
   content/external/EventUtils.js (../testing/mochitest/tests/SimpleTest/EventUtils.js)
diff --git a/remote/marionette/accessibility.js b/remote/marionette/accessibility.js
index afd05e148541..ea7d0c54476c 100644
--- a/remote/marionette/accessibility.js
+++ b/remote/marionette/accessibility.js
@@ -12,9 +12,9 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/marionette/action.js b/remote/marionette/action.js
index e2230cbe2ebe..531a52ab4362 100644
--- a/remote/marionette/action.js
+++ b/remote/marionette/action.js
@@ -15,15 +15,18 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  keyData: "chrome://remote/content/shared/webdriver/KeyData.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  AppInfo: "chrome://remote/content/shared/AppInfo.jsm",
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
   element: "chrome://remote/content/marionette/element.js",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
   event: "chrome://remote/content/marionette/event.js",
-  keyData: "chrome://remote/content/shared/webdriver/KeyData.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
-  pprint: "chrome://remote/content/shared/Format.jsm",
   Sleep: "chrome://remote/content/marionette/sync.js",
 });
 
diff --git a/remote/marionette/actors/MarionetteCommandsChild.jsm b/remote/marionette/actors/MarionetteCommandsChild.jsm
index c7d3af356256..29f01ec797a0 100644
--- a/remote/marionette/actors/MarionetteCommandsChild.jsm
+++ b/remote/marionette/actors/MarionetteCommandsChild.jsm
@@ -14,16 +14,19 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   action: "chrome://remote/content/marionette/action.js",
   atom: "chrome://remote/content/marionette/atom.js",
   element: "chrome://remote/content/marionette/element.js",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
   evaluate: "chrome://remote/content/marionette/evaluate.js",
   event: "chrome://remote/content/marionette/event.js",
   interaction: "chrome://remote/content/marionette/interaction.js",
   legacyaction: "chrome://remote/content/marionette/legacyaction.js",
-  Log: "chrome://remote/content/shared/Log.jsm",
   sandbox: "chrome://remote/content/marionette/evaluate.js",
   Sandboxes: "chrome://remote/content/marionette/evaluate.js",
 });
diff --git a/remote/marionette/actors/MarionetteCommandsParent.jsm b/remote/marionette/actors/MarionetteCommandsParent.jsm
index 78ef01b276a1..0a2c598282c8 100644
--- a/remote/marionette/actors/MarionetteCommandsParent.jsm
+++ b/remote/marionette/actors/MarionetteCommandsParent.jsm
@@ -18,12 +18,15 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   capture: "chrome://remote/content/marionette/capture.js",
   element: "chrome://remote/content/marionette/element.js",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
   evaluate: "chrome://remote/content/marionette/evaluate.js",
-  Log: "chrome://remote/content/shared/Log.jsm",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/marionette/actors/MarionetteEventsChild.jsm b/remote/marionette/actors/MarionetteEventsChild.jsm
index dbcba23046fb..c6d9626ff0be 100644
--- a/remote/marionette/actors/MarionetteEventsChild.jsm
+++ b/remote/marionette/actors/MarionetteEventsChild.jsm
@@ -14,9 +14,12 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   event: "chrome://remote/content/marionette/event.js",
-  Log: "chrome://remote/content/shared/Log.jsm",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/marionette/actors/MarionetteEventsParent.jsm b/remote/marionette/actors/MarionetteEventsParent.jsm
index 39550e6f59a5..4da2d21e8544 100644
--- a/remote/marionette/actors/MarionetteEventsParent.jsm
+++ b/remote/marionette/actors/MarionetteEventsParent.jsm
@@ -17,9 +17,12 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   EventEmitter: "resource://gre/modules/EventEmitter.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/marionette/actors/MarionetteReftestChild.jsm b/remote/marionette/actors/MarionetteReftestChild.jsm
index aec83a039823..6d27aaaac6b7 100644
--- a/remote/marionette/actors/MarionetteReftestChild.jsm
+++ b/remote/marionette/actors/MarionetteReftestChild.jsm
@@ -12,8 +12,8 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  Log: "chrome://remote/content/shared/Log.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/marionette/addon.js b/remote/marionette/addon.js
index acac4aa337e7..1c26e23dbdff 100644
--- a/remote/marionette/addon.js
+++ b/remote/marionette/addon.js
@@ -14,11 +14,12 @@ const lazy = {};
 
 ChromeUtils.defineESModuleGetters(lazy, {
   FileUtils: "resource://gre/modules/FileUtils.sys.mjs",
+
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
 });
 
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   AddonManager: "resource://gre/modules/AddonManager.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
 });
 
 // from https://developer.mozilla.org/en-US/Add-ons/Add-on_Manager/AddonManager#AddonInstall_errors
diff --git a/remote/marionette/browser.js b/remote/marionette/browser.js
index bcc56e08b811..6f22ea650b90 100644
--- a/remote/marionette/browser.js
+++ b/remote/marionette/browser.js
@@ -12,14 +12,17 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  EventPromise: "chrome://remote/content/shared/Sync.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+  windowManager: "chrome://remote/content/shared/WindowManager.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  AppInfo: "chrome://remote/content/shared/AppInfo.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  EventPromise: "chrome://remote/content/shared/Sync.jsm",
   MessageManagerDestroyedPromise: "chrome://remote/content/marionette/sync.js",
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
   WebElementEventTarget: "chrome://remote/content/marionette/dom.js",
-  windowManager: "chrome://remote/content/shared/WindowManager.jsm",
 });
 
 /** @namespace */
diff --git a/remote/marionette/capture.js b/remote/marionette/capture.js
index f7eb5224c7f6..5118fbfde9af 100644
--- a/remote/marionette/capture.js
+++ b/remote/marionette/capture.js
@@ -12,8 +12,8 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  Log: "chrome://remote/content/shared/Log.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/marionette/cookie.js b/remote/marionette/cookie.js
index 1bfbb5822de1..622b454211b4 100644
--- a/remote/marionette/cookie.js
+++ b/remote/marionette/cookie.js
@@ -6,16 +6,12 @@
 
 const EXPORTED_SYMBOLS = ["cookie"];
 
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  pprint: "chrome://remote/content/shared/Format.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
 });
 
 const IPV4_PORT_EXPR = /:\d+$/;
diff --git a/remote/marionette/dom.js b/remote/marionette/dom.js
index 80507dc411e2..49298806ede7 100644
--- a/remote/marionette/dom.js
+++ b/remote/marionette/dom.js
@@ -15,8 +15,8 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  Log: "chrome://remote/content/shared/Log.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/marionette/driver.js b/remote/marionette/driver.js
index 220089239ce3..ddf248ac0627 100644
--- a/remote/marionette/driver.js
+++ b/remote/marionette/driver.js
@@ -16,10 +16,26 @@ const { element } = ChromeUtils.import(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  EventPromise: "chrome://remote/content/shared/Sync.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
+  print: "chrome://remote/content/shared/PDF.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+  Timeouts: "chrome://remote/content/shared/webdriver/Capabilities.sys.mjs",
+  UnhandledPromptBehavior:
+    "chrome://remote/content/shared/webdriver/Capabilities.sys.mjs",
+  waitForInitialNavigationCompleted:
+    "chrome://remote/content/shared/Navigate.sys.mjs",
+  WebDriverSession: "chrome://remote/content/shared/webdriver/Session.sys.mjs",
+  windowManager: "chrome://remote/content/shared/WindowManager.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   Addon: "chrome://remote/content/marionette/addon.js",
-  AppInfo: "chrome://remote/content/shared/AppInfo.jsm",
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
   atom: "chrome://remote/content/marionette/atom.js",
   browser: "chrome://remote/content/marionette/browser.js",
   capture: "chrome://remote/content/marionette/capture.js",
@@ -32,37 +48,24 @@ XPCOMUtils.defineLazyModuleGetters(lazy, {
     "chrome://remote/content/marionette/actors/MarionetteEventsParent.jsm",
   enableEventsActor:
     "chrome://remote/content/marionette/actors/MarionetteEventsParent.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  EventPromise: "chrome://remote/content/shared/Sync.jsm",
   getMarionetteCommandsActorProxy:
     "chrome://remote/content/marionette/actors/MarionetteCommandsParent.jsm",
   IdlePromise: "chrome://remote/content/marionette/sync.js",
   l10n: "chrome://remote/content/marionette/l10n.js",
-  Log: "chrome://remote/content/shared/Log.jsm",
   Marionette: "chrome://remote/content/components/Marionette.jsm",
   MarionettePrefs: "chrome://remote/content/marionette/prefs.js",
   modal: "chrome://remote/content/marionette/modal.js",
   navigate: "chrome://remote/content/marionette/navigate.js",
   permissions: "chrome://remote/content/marionette/permissions.js",
-  pprint: "chrome://remote/content/shared/Format.jsm",
-  print: "chrome://remote/content/shared/PDF.jsm",
   reftest: "chrome://remote/content/marionette/reftest.js",
   registerCommandsActor:
     "chrome://remote/content/marionette/actors/MarionetteCommandsParent.jsm",
   RemoteAgent: "chrome://remote/content/components/RemoteAgent.jsm",
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
   TimedPromise: "chrome://remote/content/marionette/sync.js",
-  Timeouts: "chrome://remote/content/shared/webdriver/Capabilities.jsm",
-  UnhandledPromptBehavior:
-    "chrome://remote/content/shared/webdriver/Capabilities.jsm",
   unregisterCommandsActor:
     "chrome://remote/content/marionette/actors/MarionetteCommandsParent.jsm",
-  waitForInitialNavigationCompleted:
-    "chrome://remote/content/shared/Navigate.jsm",
   waitForObserverTopic: "chrome://remote/content/marionette/sync.js",
-  WebDriverSession: "chrome://remote/content/shared/webdriver/Session.jsm",
   WebReference: "chrome://remote/content/marionette/element.js",
-  windowManager: "chrome://remote/content/shared/WindowManager.jsm",
   WindowState: "chrome://remote/content/marionette/browser.js",
 });
 
diff --git a/remote/marionette/element.js b/remote/marionette/element.js
index 458581dd1f20..be31857ea2af 100644
--- a/remote/marionette/element.js
+++ b/remote/marionette/element.js
@@ -19,14 +19,17 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   ContentDOMReference: "resource://gre/modules/ContentDOMReference.jsm",
 
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
   atom: "chrome://remote/content/marionette/atom.js",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
   PollPromise: "chrome://remote/content/marionette/sync.js",
-  pprint: "chrome://remote/content/shared/Format.jsm",
 });
 
 const ORDERED_NODE_ITERATOR_TYPE = 5;
diff --git a/remote/marionette/evaluate.js b/remote/marionette/evaluate.js
index c1923d66294d..7c795e381c9b 100644
--- a/remote/marionette/evaluate.js
+++ b/remote/marionette/evaluate.js
@@ -15,10 +15,13 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   element: "chrome://remote/content/marionette/element.js",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
   WebReference: "chrome://remote/content/marionette/element.js",
 });
 
diff --git a/remote/marionette/event.js b/remote/marionette/event.js
index b6165120eb2e..ab881d009eb9 100644
--- a/remote/marionette/event.js
+++ b/remote/marionette/event.js
@@ -13,8 +13,8 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  keyData: "chrome://remote/content/shared/webdriver/KeyData.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  keyData: "chrome://remote/content/shared/webdriver/KeyData.sys.mjs",
 });
 
 /** Provides functionality for creating and sending DOM events. */
diff --git a/remote/marionette/interaction.js b/remote/marionette/interaction.js
index d28ccc9268d0..d4f67beadd2f 100644
--- a/remote/marionette/interaction.js
+++ b/remote/marionette/interaction.js
@@ -14,14 +14,17 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   accessibility: "chrome://remote/content/marionette/accessibility.js",
   atom: "chrome://remote/content/marionette/atom.js",
   element: "chrome://remote/content/marionette/element.js",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
   event: "chrome://remote/content/marionette/event.js",
-  Log: "chrome://remote/content/shared/Log.jsm",
-  pprint: "chrome://remote/content/shared/Format.jsm",
   TimedPromise: "chrome://remote/content/marionette/sync.js",
 });
 
diff --git a/remote/marionette/l10n.js b/remote/marionette/l10n.js
index eb27bdd187dd..32e96226cfa5 100644
--- a/remote/marionette/l10n.js
+++ b/remote/marionette/l10n.js
@@ -23,8 +23,8 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "domParser", () => {
diff --git a/remote/marionette/legacyaction.js b/remote/marionette/legacyaction.js
index 36644947c501..ef227a2dd1a1 100644
--- a/remote/marionette/legacyaction.js
+++ b/remote/marionette/legacyaction.js
@@ -14,15 +14,18 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   Preferences: "resource://gre/modules/Preferences.jsm",
 
   accessibility: "chrome://remote/content/marionette/accessibility.js",
   element: "chrome://remote/content/marionette/element.js",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
   evaluate: "chrome://remote/content/marionette/evaluate.js",
   event: "chrome://remote/content/marionette/event.js",
-  Log: "chrome://remote/content/shared/Log.jsm",
   WebReference: "chrome://remote/content/marionette/element.js",
 });
 
diff --git a/remote/marionette/message.js b/remote/marionette/message.js
index 6543c76e1859..42387f6e0f02 100644
--- a/remote/marionette/message.js
+++ b/remote/marionette/message.js
@@ -6,16 +6,12 @@
 
 const EXPORTED_SYMBOLS = ["Command", "Message", "Response"];
 
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  truncate: "chrome://remote/content/shared/Format.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  truncate: "chrome://remote/content/shared/Format.sys.mjs",
 });
 
 /** Representation of the packets transproted over the wire. */
diff --git a/remote/marionette/modal.js b/remote/marionette/modal.js
index c557efc7782c..a3b1494bed62 100644
--- a/remote/marionette/modal.js
+++ b/remote/marionette/modal.js
@@ -12,10 +12,9 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  AppInfo: "chrome://remote/content/shared/AppInfo.jsm",
-
-  Log: "chrome://remote/content/shared/Log.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/marionette/navigate.js b/remote/marionette/navigate.js
index 82a5e203df0f..c0a3f2ec6f29 100644
--- a/remote/marionette/navigate.js
+++ b/remote/marionette/navigate.js
@@ -12,16 +12,20 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  PageLoadStrategy:
+    "chrome://remote/content/shared/webdriver/Capabilities.sys.mjs",
+  ProgressListener: "chrome://remote/content/shared/Navigate.sys.mjs",
+  truncate: "chrome://remote/content/shared/Format.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
   EventDispatcher:
     "chrome://remote/content/marionette/actors/MarionetteEventsParent.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
   modal: "chrome://remote/content/marionette/modal.js",
-  PageLoadStrategy: "chrome://remote/content/shared/webdriver/Capabilities.jsm",
-  ProgressListener: "chrome://remote/content/shared/Navigate.jsm",
   TimedPromise: "chrome://remote/content/marionette/sync.js",
-  truncate: "chrome://remote/content/shared/Format.jsm",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/marionette/permissions.js b/remote/marionette/permissions.js
index 44fa445de74b..bf49c699d257 100644
--- a/remote/marionette/permissions.js
+++ b/remote/marionette/permissions.js
@@ -12,8 +12,11 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
   MarionettePrefs: "chrome://remote/content/marionette/prefs.js",
 });
 
diff --git a/remote/marionette/reftest.js b/remote/marionette/reftest.js
index 0ab40e1bff90..f8a31ecc29a6 100644
--- a/remote/marionette/reftest.js
+++ b/remote/marionette/reftest.js
@@ -12,16 +12,19 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  print: "chrome://remote/content/shared/PDF.sys.mjs",
+  windowManager: "chrome://remote/content/shared/WindowManager.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   E10SUtils: "resource://gre/modules/E10SUtils.jsm",
 
-  AppInfo: "chrome://remote/content/shared/AppInfo.jsm",
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
   capture: "chrome://remote/content/marionette/capture.js",
-  Log: "chrome://remote/content/shared/Log.jsm",
   navigate: "chrome://remote/content/marionette/navigate.js",
-  print: "chrome://remote/content/shared/PDF.jsm",
-  windowManager: "chrome://remote/content/shared/WindowManager.jsm",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/marionette/server.js b/remote/marionette/server.js
index 190471dcc528..09b6f36d80f2 100644
--- a/remote/marionette/server.js
+++ b/remote/marionette/server.js
@@ -12,13 +12,16 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
   Command: "chrome://remote/content/marionette/message.js",
   DebuggerTransport: "chrome://remote/content/marionette/transport.js",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
   GeckoDriver: "chrome://remote/content/marionette/driver.js",
-  Log: "chrome://remote/content/shared/Log.jsm",
   MarionettePrefs: "chrome://remote/content/marionette/prefs.js",
   Message: "chrome://remote/content/marionette/message.js",
   Response: "chrome://remote/content/marionette/message.js",
diff --git a/remote/marionette/sync.js b/remote/marionette/sync.js
index 79b86bf8bd88..c05705c72824 100644
--- a/remote/marionette/sync.js
+++ b/remote/marionette/sync.js
@@ -25,9 +25,9 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/marionette/test/xpcshell/test_message.js b/remote/marionette/test/xpcshell/test_message.js
index 206e33681cdb..8cc0f25de268 100644
--- a/remote/marionette/test/xpcshell/test_message.js
+++ b/remote/marionette/test/xpcshell/test_message.js
@@ -2,8 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-const { error } = ChromeUtils.import(
-  "chrome://remote/content/shared/webdriver/Errors.jsm"
+const { error } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Errors.sys.mjs"
 );
 const { Command, Message, Response } = ChromeUtils.import(
   "chrome://remote/content/marionette/message.js"
diff --git a/remote/server/WebSocketHandshake.jsm b/remote/server/WebSocketHandshake.jsm
index 7e6725411ee9..cc35fa3c7421 100644
--- a/remote/server/WebSocketHandshake.jsm
+++ b/remote/server/WebSocketHandshake.jsm
@@ -16,9 +16,12 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  executeSoon: "chrome://remote/content/shared/Sync.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  executeSoon: "chrome://remote/content/shared/Sync.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
   RemoteAgent: "chrome://remote/content/components/RemoteAgent.jsm",
 });
 
diff --git a/remote/shared/AppInfo.jsm b/remote/shared/AppInfo.sys.mjs
similarity index 90%
rename from remote/shared/AppInfo.jsm
rename to remote/shared/AppInfo.sys.mjs
index 5501f22e97f7..f47046fe1ca1 100644
--- a/remote/shared/AppInfo.jsm
+++ b/remote/shared/AppInfo.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["AppInfo"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const ID_FIREFOX = "{ec8030f7-c20a-464f-9b0e-13a3a9e97384}";
 const ID_THUNDERBIRD = "{3550f703-e582-4d05-9a08-453d09bdfdc6}";
@@ -27,7 +21,7 @@ const ID_THUNDERBIRD = "{3550f703-e582-4d05-9a08-453d09bdfdc6}";
  *
  * @since 88
  */
-const AppInfo = new Proxy(
+export const AppInfo = new Proxy(
   {},
   {
     get(target, prop, receiver) {
diff --git a/remote/shared/Format.jsm b/remote/shared/Format.sys.mjs
similarity index 93%
rename from remote/shared/Format.jsm
rename to remote/shared/Format.sys.mjs
index f71d97d06b68..95e9ff339f46 100644
--- a/remote/shared/Format.jsm
+++ b/remote/shared/Format.sys.mjs
@@ -2,18 +2,12 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-var EXPORTED_SYMBOLS = ["pprint", "truncate"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  Log: "chrome://remote/content/shared/Log.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
@@ -44,7 +38,7 @@ const MAX_STRING_LENGTH = 250;
  *     pprint`Current window: ${window}`;
  *     => '[object Window https://www.mozilla.org/]'
  */
-function pprint(ss, ...values) {
+export function pprint(ss, ...values) {
   function pretty(val) {
     let proto = Object.prototype.toString.call(val);
     if (
@@ -136,7 +130,7 @@ function pprint(ss, ...values) {
  *
  * Functions named `toJSON` or `toString` on objects will be called.
  */
-function truncate(strings, ...values) {
+export function truncate(strings, ...values) {
   function walk(obj) {
     const typ = Object.prototype.toString.call(obj);
 
diff --git a/remote/shared/Log.jsm b/remote/shared/Log.sys.mjs
similarity index 91%
rename from remote/shared/Log.jsm
rename to remote/shared/Log.sys.mjs
index 6c2a138e016e..fba951fa97e8 100644
--- a/remote/shared/Log.jsm
+++ b/remote/shared/Log.sys.mjs
@@ -2,16 +2,9 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
-var EXPORTED_SYMBOLS = ["Log"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-const { Log: StdLog } = ChromeUtils.importESModule(
-  "resource://gre/modules/Log.sys.mjs"
-);
+const { Log: StdLog } = ChromeUtils.import("resource://gre/modules/Log.jsm");
 
 const PREF_REMOTE_LOG_LEVEL = "remote.log.level";
 
@@ -42,7 +35,7 @@ XPCOMUtils.defineLazyGetter(lazy, "prefLogLevel", () => {
 });
 
 /** E10s compatible wrapper for the standard logger from Log.jsm. */
-class Log {
+export class Log {
   static TYPES = {
     CDP: "CDP",
     MARIONETTE: "Marionette",
diff --git a/remote/shared/MobileTabBrowser.jsm b/remote/shared/MobileTabBrowser.sys.mjs
similarity index 88%
rename from remote/shared/MobileTabBrowser.jsm
rename to remote/shared/MobileTabBrowser.sys.mjs
index 82bf4b336bf1..fd8e78e45ada 100644
--- a/remote/shared/MobileTabBrowser.jsm
+++ b/remote/shared/MobileTabBrowser.sys.mjs
@@ -2,24 +2,20 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-var EXPORTED_SYMBOLS = ["MobileTabBrowser"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  windowManager: "chrome://remote/content/shared/WindowManager.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   GeckoViewTabUtil: "resource://gre/modules/GeckoViewTestUtils.jsm",
-
-  windowManager: "chrome://remote/content/shared/WindowManager.jsm",
 });
 
 // GeckoView shim for Desktop's gBrowser
-class MobileTabBrowser {
+export class MobileTabBrowser {
   constructor(window) {
     this.window = window;
   }
diff --git a/remote/shared/Navigate.jsm b/remote/shared/Navigate.sys.mjs
similarity index 96%
rename from remote/shared/Navigate.jsm
rename to remote/shared/Navigate.sys.mjs
index 2b46360b8ade..ec9218414742 100644
--- a/remote/shared/Navigate.jsm
+++ b/remote/shared/Navigate.sys.mjs
@@ -2,29 +2,23 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = [
-  "ProgressListener",
-  "waitForInitialNavigationCompleted",
-];
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const { AppConstants } = ChromeUtils.import(
   "resource://gre/modules/AppConstants.jsm"
 );
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  Deferred: "chrome://remote/content/shared/Sync.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  truncate: "chrome://remote/content/shared/Format.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   clearTimeout: "resource://gre/modules/Timer.jsm",
   setTimeout: "resource://gre/modules/Timer.jsm",
-
-  Deferred: "chrome://remote/content/shared/Sync.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
-  truncate: "chrome://remote/content/shared/Format.jsm",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
@@ -69,7 +63,10 @@ const webProgressListeners = new Set();
  *       - {nsIURI} currentURI The current URI of the page
  *       - {nsIURI} targetURI Target URI of the navigation
  */
-async function waitForInitialNavigationCompleted(webProgress, options = {}) {
+export async function waitForInitialNavigationCompleted(
+  webProgress,
+  options = {}
+) {
   const { resolveWhenStarted = false } = options;
 
   const browsingContext = webProgress.browsingContext;
@@ -110,7 +107,7 @@ async function waitForInitialNavigationCompleted(webProgress, options = {}) {
 /**
  * WebProgressListener to observe for page loads.
  */
-class ProgressListener {
+export class ProgressListener {
   #expectNavigation;
   #resolveWhenStarted;
   #unloadTimeout;
diff --git a/remote/shared/PDF.jsm b/remote/shared/PDF.sys.mjs
similarity index 95%
rename from remote/shared/PDF.jsm
rename to remote/shared/PDF.sys.mjs
index 21a21d6386c1..3eef525ec90d 100644
--- a/remote/shared/PDF.jsm
+++ b/remote/shared/PDF.sys.mjs
@@ -2,27 +2,23 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["print"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
   clearInterval: "resource://gre/modules/Timer.jsm",
   setInterval: "resource://gre/modules/Timer.jsm",
-
-  Log: "chrome://remote/content/shared/Log.jsm",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
 
-const print = {
+export const print = {
   maxScaleValue: 2.0,
   minScaleValue: 0.1,
   letterPaperSizeCm: {
diff --git a/remote/shared/RecommendedPreferences.jsm b/remote/shared/RecommendedPreferences.sys.mjs
similarity index 97%
rename from remote/shared/RecommendedPreferences.jsm
rename to remote/shared/RecommendedPreferences.sys.mjs
index 9b67a573799c..746b892ff9ae 100644
--- a/remote/shared/RecommendedPreferences.jsm
+++ b/remote/shared/RecommendedPreferences.sys.mjs
@@ -2,20 +2,16 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["RecommendedPreferences"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   Preferences: "resource://gre/modules/Preferences.jsm",
-
-  Log: "chrome://remote/content/shared/Log.jsm",
 });
 
 XPCOMUtils.defineLazyPreferenceGetter(
@@ -271,7 +267,7 @@ const COMMON_PREFERENCES = new Map([
   ["browser.topsites.contile.enabled", false],
 ]);
 
-const RecommendedPreferences = {
+export const RecommendedPreferences = {
   alteredPrefs: new Set(),
 
   isInitialized: false,
diff --git a/remote/shared/RemoteError.jsm b/remote/shared/RemoteError.sys.mjs
similarity index 83%
rename from remote/shared/RemoteError.jsm
rename to remote/shared/RemoteError.sys.mjs
index a18c81b9cff9..2e326d5d1835 100644
--- a/remote/shared/RemoteError.jsm
+++ b/remote/shared/RemoteError.sys.mjs
@@ -2,14 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["RemoteError"];
-
 /**
  * Base class for all remote protocol errors.
  */
-class RemoteError extends Error {
+export class RemoteError extends Error {
   get isRemoteError() {
     return true;
   }
diff --git a/remote/shared/Stack.jsm b/remote/shared/Stack.sys.mjs
similarity index 92%
rename from remote/shared/Stack.jsm
rename to remote/shared/Stack.sys.mjs
index de0f537d7913..4611dbb04fd4 100644
--- a/remote/shared/Stack.jsm
+++ b/remote/shared/Stack.sys.mjs
@@ -2,10 +2,6 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["getFramesFromStack", "isChromeFrame"];
-
 /**
  * An object that contains details of a stack frame.
  *
@@ -36,7 +32,7 @@ const EXPORTED_SYMBOLS = ["getFramesFromStack", "isChromeFrame"];
  *
  * @returns {Array<StackFrame>=}
  */
-function getFramesFromStack(stack) {
+export function getFramesFromStack(stack) {
   if (!stack || (Cu && Cu.isDeadWrapper(stack))) {
     // If the global from which this error came from has been nuked,
     // stack is going to be a dead wrapper.
@@ -69,7 +65,7 @@ function getFramesFromStack(stack) {
  * @returns {boolean}
  *     True, if frame is from chrome scope
  */
-function isChromeFrame(frame) {
+export function isChromeFrame(frame) {
   return (
     frame.filename.startsWith("chrome://") ||
     frame.filename.startsWith("resource://")
diff --git a/remote/shared/Sync.jsm b/remote/shared/Sync.sys.mjs
similarity index 93%
rename from remote/shared/Sync.jsm
rename to remote/shared/Sync.sys.mjs
index 8c68e73a2913..c05f27a86875 100644
--- a/remote/shared/Sync.jsm
+++ b/remote/shared/Sync.sys.mjs
@@ -2,25 +2,13 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-var EXPORTED_SYMBOLS = [
-  "AnimationFramePromise",
-  "Deferred",
-  "EventPromise",
-  "executeSoon",
-  "PollPromise",
-];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
 });
 
 const { TYPE_ONE_SHOT, TYPE_REPEATING_SLACK } = Ci.nsITimer;
@@ -37,7 +25,7 @@ XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
  *
  * @return {Promise}
  */
-function AnimationFramePromise(win) {
+export function AnimationFramePromise(win) {
   const animationFramePromise = new Promise(resolve => {
     win.requestAnimationFrame(resolve);
   });
@@ -64,7 +52,7 @@ function AnimationFramePromise(win) {
  *       - rejected Flag that indicates that the promise got rejected
  *       - resolve Callback to resolve the promise
  */
-function Deferred() {
+export function Deferred() {
   const deferred = {};
 
   deferred.promise = new Promise((resolve, reject) => {
@@ -133,7 +121,7 @@ function Deferred() {
  * @throws {TypeError}
  * @throws {RangeError}
  */
-function EventPromise(subject, eventName, options = {}) {
+export function EventPromise(subject, eventName, options = {}) {
   const {
     capture = false,
     checkFn = null,
@@ -210,7 +198,7 @@ function EventPromise(subject, eventName, options = {}) {
  * @param {function} fn
  *     Function to be executed.
  */
-function executeSoon(fn) {
+export function executeSoon(fn) {
   if (typeof fn != "function") {
     throw new TypeError();
   }
@@ -269,7 +257,7 @@ function executeSoon(fn) {
  * @throws {RangeError}
  *     If `timeout` or `interval` are not unsigned integers.
  */
-function PollPromise(func, { timeout = null, interval = 10 } = {}) {
+export function PollPromise(func, { timeout = null, interval = 10 } = {}) {
   const timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
 
   if (typeof func != "function") {
diff --git a/remote/shared/TabManager.jsm b/remote/shared/TabManager.sys.mjs
similarity index 95%
rename from remote/shared/TabManager.jsm
rename to remote/shared/TabManager.sys.mjs
index f970c2c92a17..9123968a6867 100644
--- a/remote/shared/TabManager.jsm
+++ b/remote/shared/TabManager.sys.mjs
@@ -2,26 +2,18 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-var EXPORTED_SYMBOLS = ["TabManager"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  AppInfo: "chrome://remote/content/shared/AppInfo.jsm",
-  EventPromise: "chrome://remote/content/shared/Sync.jsm",
-  MobileTabBrowser: "chrome://remote/content/shared/MobileTabBrowser.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  EventPromise: "chrome://remote/content/shared/Sync.sys.mjs",
+  MobileTabBrowser: "chrome://remote/content/shared/MobileTabBrowser.sys.mjs",
 });
 
 // Maps browser's permanentKey to uuid: WeakMap.<Object, string>
 const browserUniqueIds = new WeakMap();
 
-var TabManager = {
+export var TabManager = {
   /**
    * Retrieve all the browser elements from tabs as contained in open windows.
    *
diff --git a/remote/shared/WebSocketConnection.jsm b/remote/shared/WebSocketConnection.sys.mjs
similarity index 91%
rename from remote/shared/WebSocketConnection.jsm
rename to remote/shared/WebSocketConnection.sys.mjs
index d2a102db517c..b865725acc73 100644
--- a/remote/shared/WebSocketConnection.jsm
+++ b/remote/shared/WebSocketConnection.sys.mjs
@@ -3,25 +3,22 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-var EXPORTED_SYMBOLS = ["WebSocketConnection"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  truncate: "chrome://remote/content/shared/Format.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  Log: "chrome://remote/content/shared/Log.jsm",
-  truncate: "chrome://remote/content/shared/Format.jsm",
   WebSocketTransport: "chrome://remote/content/server/WebSocketTransport.jsm",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
 
-class WebSocketConnection {
+export class WebSocketConnection {
   /**
    * @param {WebSocket} webSocket
    *     The WebSocket server connection to wrap.
diff --git a/remote/shared/WindowManager.jsm b/remote/shared/WindowManager.sys.mjs
similarity index 95%
rename from remote/shared/WindowManager.jsm
rename to remote/shared/WindowManager.sys.mjs
index 28943a438663..24a82331a0f2 100644
--- a/remote/shared/WindowManager.jsm
+++ b/remote/shared/WindowManager.sys.mjs
@@ -2,22 +2,19 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["windowManager"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  EventPromise: "chrome://remote/content/shared/Sync.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  AppInfo: "chrome://remote/content/shared/AppInfo.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
   TimedPromise: "chrome://remote/content/marionette/sync.js",
-  EventPromise: "chrome://remote/content/shared/Sync.jsm",
   waitForObserverTopic: "chrome://remote/content/marionette/sync.js",
 });
 
@@ -279,4 +276,4 @@ class WindowManager {
 }
 
 // Expose a shared singleton.
-const windowManager = new WindowManager();
+export const windowManager = new WindowManager();
diff --git a/remote/shared/listeners/BrowsingContextListener.jsm b/remote/shared/listeners/BrowsingContextListener.sys.mjs
similarity index 94%
rename from remote/shared/listeners/BrowsingContextListener.jsm
rename to remote/shared/listeners/BrowsingContextListener.sys.mjs
index ccd797ef093d..a35a210161c6 100644
--- a/remote/shared/listeners/BrowsingContextListener.jsm
+++ b/remote/shared/listeners/BrowsingContextListener.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["BrowsingContextListener"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
@@ -46,7 +40,7 @@ const OBSERVER_TOPIC_SET_EMBEDDER = "browsing-context-did-set-embedder";
  *            Usually "attach" or "discard", but will contain "replace" if the
  *            browsing context gets replaced by a cross-group navigation.
  */
-class BrowsingContextListener {
+export class BrowsingContextListener {
   #listening;
   #topContextsToAttach;
 
diff --git a/remote/shared/listeners/ConsoleAPIListener.jsm b/remote/shared/listeners/ConsoleAPIListener.sys.mjs
similarity index 94%
rename from remote/shared/listeners/ConsoleAPIListener.jsm
rename to remote/shared/listeners/ConsoleAPIListener.sys.mjs
index 079f03d256b5..f4fcce213b64 100644
--- a/remote/shared/listeners/ConsoleAPIListener.jsm
+++ b/remote/shared/listeners/ConsoleAPIListener.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["ConsoleAPIListener"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
@@ -46,7 +40,7 @@ XPCOMUtils.defineLazyGetter(lazy, "ConsoleAPIStorage", () => {
  *      - {Array<Object>} stacktrace - List of stack frames, starting from most recent.
  *      - {Number} timeStamp - Timestamp when the method was called.
  */
-class ConsoleAPIListener {
+export class ConsoleAPIListener {
   #emittedMessages;
   #innerWindowId;
   #listening;
diff --git a/remote/shared/listeners/ConsoleListener.jsm b/remote/shared/listeners/ConsoleListener.sys.mjs
similarity index 93%
rename from remote/shared/listeners/ConsoleListener.jsm
rename to remote/shared/listeners/ConsoleListener.sys.mjs
index 7034b30f8c36..cb347dae063b 100644
--- a/remote/shared/listeners/ConsoleListener.jsm
+++ b/remote/shared/listeners/ConsoleListener.sys.mjs
@@ -2,21 +2,17 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["ConsoleListener"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  getFramesFromStack: "chrome://remote/content/shared/Stack.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   EventEmitter: "resource://gre/modules/EventEmitter.jsm",
-
-  getFramesFromStack: "chrome://remote/content/shared/Stack.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
@@ -51,7 +47,7 @@ XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
  *        starting from most recent.
  *      - {Number} timeStamp - Timestamp when the method was called.
  */
-class ConsoleListener {
+export class ConsoleListener {
   #emittedMessages;
   #innerWindowId;
   #listening;
diff --git a/remote/shared/listeners/LoadListener.jsm b/remote/shared/listeners/LoadListener.sys.mjs
similarity index 91%
rename from remote/shared/listeners/LoadListener.jsm
rename to remote/shared/listeners/LoadListener.sys.mjs
index f5e70d4a128f..65305d789303 100644
--- a/remote/shared/listeners/LoadListener.jsm
+++ b/remote/shared/listeners/LoadListener.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["LoadListener"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
@@ -37,7 +31,7 @@ XPCOMUtils.defineLazyModuleGetters(lazy, {
  *      - {Document} target
  *            The target document.
  */
-class LoadListener {
+export class LoadListener {
   #abortController;
   #window;
 
diff --git a/remote/shared/listeners/test/browser/browser_BrowsingContextListener.js b/remote/shared/listeners/test/browser/browser_BrowsingContextListener.js
index cfbd55ac53f6..9a08df7857b6 100644
--- a/remote/shared/listeners/test/browser/browser_BrowsingContextListener.js
+++ b/remote/shared/listeners/test/browser/browser_BrowsingContextListener.js
@@ -2,8 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-const { BrowsingContextListener } = ChromeUtils.import(
-  "chrome://remote/content/shared/listeners/BrowsingContextListener.jsm"
+const { BrowsingContextListener } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/listeners/BrowsingContextListener.sys.mjs"
 );
 
 add_task(async function test_attachedOnNewTab() {
diff --git a/remote/shared/listeners/test/browser/browser_ConsoleAPIListener.js b/remote/shared/listeners/test/browser/browser_ConsoleAPIListener.js
index 827721222296..ccff78c7a08c 100644
--- a/remote/shared/listeners/test/browser/browser_ConsoleAPIListener.js
+++ b/remote/shared/listeners/test/browser/browser_ConsoleAPIListener.js
@@ -115,8 +115,8 @@ function listenToConsoleAPIMessage() {
   info("Listen to a console api message in content");
   return SpecialPowers.spawn(gBrowser.selectedBrowser, [], async () => {
     const innerWindowId = content.windowGlobalChild.innerWindowId;
-    const { ConsoleAPIListener } = ChromeUtils.import(
-      "chrome://remote/content/shared/listeners/ConsoleAPIListener.jsm"
+    const { ConsoleAPIListener } = ChromeUtils.importESModule(
+      "chrome://remote/content/shared/listeners/ConsoleAPIListener.sys.mjs"
     );
     const consoleAPIListener = new ConsoleAPIListener(innerWindowId);
     const onMessage = consoleAPIListener.once("message");
diff --git a/remote/shared/listeners/test/browser/browser_ConsoleAPIListener_cached_messages.js b/remote/shared/listeners/test/browser/browser_ConsoleAPIListener_cached_messages.js
index 8225ae5a7363..dae35a0b9ab1 100644
--- a/remote/shared/listeners/test/browser/browser_ConsoleAPIListener_cached_messages.js
+++ b/remote/shared/listeners/test/browser/browser_ConsoleAPIListener_cached_messages.js
@@ -10,8 +10,8 @@ add_task(async function test_cached_messages() {
 
   await SpecialPowers.spawn(gBrowser.selectedBrowser, [], async () => {
     const innerWindowId = content.windowGlobalChild.innerWindowId;
-    const { ConsoleAPIListener } = ChromeUtils.import(
-      "chrome://remote/content/shared/listeners/ConsoleAPIListener.jsm"
+    const { ConsoleAPIListener } = ChromeUtils.importESModule(
+      "chrome://remote/content/shared/listeners/ConsoleAPIListener.sys.mjs"
     );
 
     info("Log two messages before starting the ConsoleAPIListener");
@@ -67,8 +67,8 @@ add_task(async function test_cached_messages() {
 
   await SpecialPowers.spawn(gBrowser.selectedBrowser, [], async () => {
     const innerWindowId = content.windowGlobalChild.innerWindowId;
-    const { ConsoleAPIListener } = ChromeUtils.import(
-      "chrome://remote/content/shared/listeners/ConsoleAPIListener.jsm"
+    const { ConsoleAPIListener } = ChromeUtils.importESModule(
+      "chrome://remote/content/shared/listeners/ConsoleAPIListener.sys.mjs"
     );
 
     info("Log a message before creating the ConsoleAPIListener");
diff --git a/remote/shared/listeners/test/browser/browser_ConsoleListener.js b/remote/shared/listeners/test/browser/browser_ConsoleListener.js
index cd2e3d8981e1..7bc098ca8a49 100644
--- a/remote/shared/listeners/test/browser/browser_ConsoleListener.js
+++ b/remote/shared/listeners/test/browser/browser_ConsoleListener.js
@@ -100,8 +100,8 @@ function listenToConsoleMessage(level) {
     [level],
     async _level => {
       const innerWindowId = content.windowGlobalChild.innerWindowId;
-      const { ConsoleListener } = ChromeUtils.import(
-        "chrome://remote/content/shared/listeners/ConsoleListener.jsm"
+      const { ConsoleListener } = ChromeUtils.importESModule(
+        "chrome://remote/content/shared/listeners/ConsoleListener.sys.mjs"
       );
       const consoleListener = new ConsoleListener(innerWindowId);
       const onMessage = consoleListener.once(_level);
diff --git a/remote/shared/listeners/test/browser/browser_ConsoleListener_cached_messages.js b/remote/shared/listeners/test/browser/browser_ConsoleListener_cached_messages.js
index c9d69dc58179..1020aee66111 100644
--- a/remote/shared/listeners/test/browser/browser_ConsoleListener_cached_messages.js
+++ b/remote/shared/listeners/test/browser/browser_ConsoleListener_cached_messages.js
@@ -12,8 +12,8 @@ add_task(async function test_cached_javascript_errors() {
   await createScriptNode(`(() => {throw "error1"})()`);
   await SpecialPowers.spawn(gBrowser.selectedBrowser, [], async () => {
     const innerWindowId = content.windowGlobalChild.innerWindowId;
-    const { ConsoleListener } = ChromeUtils.import(
-      "chrome://remote/content/shared/listeners/ConsoleListener.jsm"
+    const { ConsoleListener } = ChromeUtils.importESModule(
+      "chrome://remote/content/shared/listeners/ConsoleListener.sys.mjs"
     );
 
     const listener = new ConsoleListener(innerWindowId);
@@ -58,8 +58,8 @@ add_task(async function test_cached_javascript_errors() {
   await createScriptNode(`(() => {throw "error3"})()`);
   await SpecialPowers.spawn(gBrowser.selectedBrowser, [], async () => {
     const innerWindowId = content.windowGlobalChild.innerWindowId;
-    const { ConsoleListener } = ChromeUtils.import(
-      "chrome://remote/content/shared/listeners/ConsoleListener.jsm"
+    const { ConsoleListener } = ChromeUtils.importESModule(
+      "chrome://remote/content/shared/listeners/ConsoleListener.sys.mjs"
     );
 
     const listener = new ConsoleListener(innerWindowId);
diff --git a/remote/shared/messagehandler/Errors.jsm b/remote/shared/messagehandler/Errors.sys.mjs
similarity index 92%
rename from remote/shared/messagehandler/Errors.jsm
rename to remote/shared/messagehandler/Errors.sys.mjs
index 175c24ccef54..4e7d5400ec2c 100644
--- a/remote/shared/messagehandler/Errors.jsm
+++ b/remote/shared/messagehandler/Errors.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["error"];
-
-const { RemoteError } = ChromeUtils.import(
-  "chrome://remote/content/shared/RemoteError.jsm"
-);
+import { RemoteError } from "chrome://remote/content/shared/RemoteError.sys.mjs";
 
 class MessageHandlerError extends RemoteError {
   /**
@@ -90,7 +84,7 @@ const STATUSES = new Map([
 ]);
 
 /** @namespace */
-const error = {
+export const error = {
   MessageHandlerError,
   UnsupportedCommandError,
 };
diff --git a/remote/shared/messagehandler/EventsDispatcher.jsm b/remote/shared/messagehandler/EventsDispatcher.sys.mjs
similarity index 93%
rename from remote/shared/messagehandler/EventsDispatcher.jsm
rename to remote/shared/messagehandler/EventsDispatcher.sys.mjs
index 7e32867991ae..83d244f63018 100644
--- a/remote/shared/messagehandler/EventsDispatcher.jsm
+++ b/remote/shared/messagehandler/EventsDispatcher.sys.mjs
@@ -2,21 +2,15 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["EventsDispatcher"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
+ChromeUtils.defineESModuleGetters(lazy, {
   ContextDescriptorType:
-    "chrome://remote/content/shared/messagehandler/MessageHandler.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
+    "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
@@ -27,7 +21,7 @@ XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
  * subscribe and unsubscribe to those events based on SessionData updates
  * and should use the "event" SessionData category.
  */
-class EventsDispatcher {
+export class EventsDispatcher {
   // The MessageHandler owning this EventsDispatcher.
   #messageHandler;
 
diff --git a/remote/shared/messagehandler/MessageHandler.jsm b/remote/shared/messagehandler/MessageHandler.sys.mjs
similarity index 95%
rename from remote/shared/messagehandler/MessageHandler.jsm
rename to remote/shared/messagehandler/MessageHandler.sys.mjs
index 7988df1bd8a5..440a57de5644 100644
--- a/remote/shared/messagehandler/MessageHandler.jsm
+++ b/remote/shared/messagehandler/MessageHandler.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["ContextDescriptorType", "MessageHandler"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const { EventEmitter } = ChromeUtils.import(
   "resource://gre/modules/EventEmitter.jsm"
@@ -16,12 +10,13 @@ const { EventEmitter } = ChromeUtils.import(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  error: "chrome://remote/content/shared/messagehandler/Errors.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/messagehandler/Errors.sys.mjs",
   EventsDispatcher:
-    "chrome://remote/content/shared/messagehandler/EventsDispatcher.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
-  ModuleCache: "chrome://remote/content/shared/messagehandler/ModuleCache.jsm",
+    "chrome://remote/content/shared/messagehandler/EventsDispatcher.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  ModuleCache:
+    "chrome://remote/content/shared/messagehandler/ModuleCache.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
@@ -45,7 +40,7 @@ XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
  *
  * @enum {string}
  */
-const ContextDescriptorType = {
+export const ContextDescriptorType = {
   All: "All",
   TopBrowsingContext: "TopBrowsingContext",
 };
@@ -90,7 +85,7 @@ const ContextDescriptorType = {
  * instances are properly registered and can be retrieved based on a given
  * session id as well as some other context information.
  */
-class MessageHandler extends EventEmitter {
+export class MessageHandler extends EventEmitter {
   #context;
   #contextId;
   #eventsDispatcher;
diff --git a/remote/shared/messagehandler/MessageHandlerRegistry.jsm b/remote/shared/messagehandler/MessageHandlerRegistry.sys.mjs
similarity index 94%
rename from remote/shared/messagehandler/MessageHandlerRegistry.jsm
rename to remote/shared/messagehandler/MessageHandlerRegistry.sys.mjs
index eacf97235e51..8b9cebf8f0a6 100644
--- a/remote/shared/messagehandler/MessageHandlerRegistry.jsm
+++ b/remote/shared/messagehandler/MessageHandlerRegistry.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["getMessageHandlerClass", "MessageHandlerRegistry"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const { EventEmitter } = ChromeUtils.import(
   "resource://gre/modules/EventEmitter.jsm"
@@ -16,14 +10,14 @@ const { EventEmitter } = ChromeUtils.import(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  Log: "chrome://remote/content/shared/Log.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
   readSessionData:
-    "chrome://remote/content/shared/messagehandler/sessiondata/SessionDataReader.jsm",
+    "chrome://remote/content/shared/messagehandler/sessiondata/SessionDataReader.sys.mjs",
   RootMessageHandler:
-    "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm",
+    "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs",
   WindowGlobalMessageHandler:
-    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm",
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
@@ -51,7 +45,7 @@ XPCOMUtils.defineLazyGetter(
  * @throws {Error}
  *      Throws if no MessageHandler subclass is found for the provided type.
  */
-function getMessageHandlerClass(type) {
+export function getMessageHandlerClass(type) {
   if (!lazy.MessageHandlerClasses.has(type)) {
     throw new Error(`No MessageHandler class available for type "${type}"`);
   }
@@ -70,7 +64,7 @@ function getMessageHandlerClass(type) {
  * The registry is useful to retrieve the appropriate MessageHandler instance
  * after crossing a technical boundary (eg process, thread...).
  */
-class MessageHandlerRegistry extends EventEmitter {
+export class MessageHandlerRegistry extends EventEmitter {
   /*
    * @param {String} type
    *     MessageHandler type, one of MessageHandler.type.
diff --git a/remote/shared/messagehandler/Module.jsm b/remote/shared/messagehandler/Module.sys.mjs
similarity index 93%
rename from remote/shared/messagehandler/Module.jsm
rename to remote/shared/messagehandler/Module.sys.mjs
index ca05f067ca6a..d622b73f12c3 100644
--- a/remote/shared/messagehandler/Module.jsm
+++ b/remote/shared/messagehandler/Module.sys.mjs
@@ -2,27 +2,21 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["Module"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
+ChromeUtils.defineESModuleGetters(lazy, {
   ContextDescriptorType:
-    "chrome://remote/content/shared/messagehandler/MessageHandler.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
+    "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "disabledExperimentalAPI", () => {
   return !Services.prefs.getBoolPref("remote.experimental.enabled");
 });
 
-class Module {
+export class Module {
   #messageHandler;
 
   /**
diff --git a/remote/shared/messagehandler/ModuleCache.jsm b/remote/shared/messagehandler/ModuleCache.sys.mjs
similarity index 95%
rename from remote/shared/messagehandler/ModuleCache.jsm
rename to remote/shared/messagehandler/ModuleCache.sys.mjs
index fcbfb718512b..cfce4c9c5728 100644
--- a/remote/shared/messagehandler/ModuleCache.jsm
+++ b/remote/shared/messagehandler/ModuleCache.sys.mjs
@@ -2,20 +2,14 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["ModuleCache"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
+ChromeUtils.defineESModuleGetters(lazy, {
   getMessageHandlerClass:
-    "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
+    "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
 });
 
 const protocols = {
@@ -29,9 +23,10 @@ XPCOMUtils.defineLazyModuleGetters(protocols.bidi, {
   getModuleClass:
     "chrome://remote/content/webdriver-bidi/modules/ModuleRegistry.jsm",
 });
-XPCOMUtils.defineLazyModuleGetters(protocols.test, {
+// eslint-disable-next-line mozilla/lazy-getter-object-name
+ChromeUtils.defineESModuleGetters(protocols.test, {
   getModuleClass:
-    "chrome://mochitests/content/browser/remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.jsm",
+    "chrome://mochitests/content/browser/remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
@@ -67,7 +62,7 @@ XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
  * the logic detailed above, the MessageHandler will assume that the command
  * should simply be forwarded to the next layer of the network.
  */
-class ModuleCache {
+export class ModuleCache {
   /*
    * @param {MessageHandler} messageHandler
    *     The MessageHandler instance which owns this ModuleCache instance.
diff --git a/remote/shared/messagehandler/RootMessageHandler.jsm b/remote/shared/messagehandler/RootMessageHandler.sys.mjs
similarity index 92%
rename from remote/shared/messagehandler/RootMessageHandler.jsm
rename to remote/shared/messagehandler/RootMessageHandler.sys.mjs
index 98498184e05c..ab3c82d4c3c2 100644
--- a/remote/shared/messagehandler/RootMessageHandler.jsm
+++ b/remote/shared/messagehandler/RootMessageHandler.sys.mjs
@@ -2,27 +2,17 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["RootMessageHandler"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
-const { MessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/MessageHandler.jsm"
-);
+import { MessageHandler } from "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs";
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
+ChromeUtils.defineESModuleGetters(lazy, {
   FrameTransport:
-    "chrome://remote/content/shared/messagehandler/transports/FrameTransport.jsm",
+    "chrome://remote/content/shared/messagehandler/transports/FrameTransport.sys.mjs",
   SessionData:
-    "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.jsm",
+    "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.sys.mjs",
   WindowGlobalMessageHandler:
-    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm",
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs",
 });
 
 /**
@@ -30,7 +20,7 @@ XPCOMUtils.defineLazyModuleGetters(lazy, {
  * in the parent process. It can forward commands to MessageHandlers in other
  * layers (at the moment WindowGlobalMessageHandlers in content processes).
  */
-class RootMessageHandler extends MessageHandler {
+export class RootMessageHandler extends MessageHandler {
   #frameTransport;
   #sessionData;
 
diff --git a/remote/shared/messagehandler/RootMessageHandlerRegistry.jsm b/remote/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs
similarity index 56%
rename from remote/shared/messagehandler/RootMessageHandlerRegistry.jsm
rename to remote/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs
index 91f75dcae403..09ac4891826a 100644
--- a/remote/shared/messagehandler/RootMessageHandlerRegistry.jsm
+++ b/remote/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs
@@ -2,16 +2,9 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
+import { MessageHandlerRegistry } from "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.sys.mjs";
 
-var EXPORTED_SYMBOLS = ["RootMessageHandlerRegistry"];
-
-const { MessageHandlerRegistry } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.jsm"
-);
-const { RootMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm"
-);
+import { RootMessageHandler } from "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs";
 
 /**
  * In the parent process, only one Root MessageHandlerRegistry should ever be
@@ -19,6 +12,6 @@ const { RootMessageHandler } = ChromeUtils.import(
  * registry and from there either create or retrieve Root MessageHandler
  * instances for a specific session.
  */
-var RootMessageHandlerRegistry = new MessageHandlerRegistry(
+export var RootMessageHandlerRegistry = new MessageHandlerRegistry(
   RootMessageHandler.type
 );
diff --git a/remote/shared/messagehandler/WindowGlobalMessageHandler.jsm b/remote/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs
similarity index 92%
rename from remote/shared/messagehandler/WindowGlobalMessageHandler.jsm
rename to remote/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs
index 2e15a17b9daa..3c2974050589 100644
--- a/remote/shared/messagehandler/WindowGlobalMessageHandler.jsm
+++ b/remote/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs
@@ -2,13 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["WindowGlobalMessageHandler"];
-
-const { ContextDescriptorType, MessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/MessageHandler.jsm"
-);
+import {
+  ContextDescriptorType,
+  MessageHandler,
+} from "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs";
 
 /**
  * A WindowGlobalMessageHandler is dedicated to debugging a single window
@@ -17,7 +14,7 @@ const { ContextDescriptorType, MessageHandler } = ChromeUtils.import(
  * commands further to other MessageHandlers and represents a leaf node in a
  * MessageHandler network.
  */
-class WindowGlobalMessageHandler extends MessageHandler {
+export class WindowGlobalMessageHandler extends MessageHandler {
   #innerWindowId;
 
   constructor() {
diff --git a/remote/shared/messagehandler/sessiondata/SessionData.jsm b/remote/shared/messagehandler/sessiondata/SessionData.sys.mjs
similarity index 95%
rename from remote/shared/messagehandler/sessiondata/SessionData.jsm
rename to remote/shared/messagehandler/sessiondata/SessionData.sys.mjs
index 50b0709be7ce..3c202b0a8156 100644
--- a/remote/shared/messagehandler/sessiondata/SessionData.jsm
+++ b/remote/shared/messagehandler/sessiondata/SessionData.sys.mjs
@@ -2,27 +2,21 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["SESSION_DATA_SHARED_DATA_KEY", "SessionData"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
+ChromeUtils.defineESModuleGetters(lazy, {
   ContextDescriptorType:
-    "chrome://remote/content/shared/messagehandler/MessageHandler.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
+    "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
   RootMessageHandler:
-    "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm",
+    "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
 
-const SESSION_DATA_SHARED_DATA_KEY = "MessageHandlerSessionData";
+export const SESSION_DATA_SHARED_DATA_KEY = "MessageHandlerSessionData";
 
 // This is a map from session id to session data, which will be persisted and
 // propagated to all processes using Services' sharedData.
@@ -86,7 +80,7 @@ const sessionDataMap = new Map();
  * SessionDataReader's readSessionData to get read-only access to session data.
  *
  */
-class SessionData {
+export class SessionData {
   constructor(messageHandler) {
     if (messageHandler.constructor.type != lazy.RootMessageHandler.type) {
       throw new Error(
diff --git a/remote/shared/messagehandler/sessiondata/SessionDataReader.jsm b/remote/shared/messagehandler/sessiondata/SessionDataReader.sys.mjs
similarity index 77%
rename from remote/shared/messagehandler/sessiondata/SessionDataReader.jsm
rename to remote/shared/messagehandler/sessiondata/SessionDataReader.sys.mjs
index 5d33c7ebca15..107ab82dc622 100644
--- a/remote/shared/messagehandler/sessiondata/SessionDataReader.jsm
+++ b/remote/shared/messagehandler/sessiondata/SessionDataReader.sys.mjs
@@ -2,19 +2,13 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["readSessionData"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
+ChromeUtils.defineESModuleGetters(lazy, {
   SESSION_DATA_SHARED_DATA_KEY:
-    "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.jsm",
+    "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "sharedData", () => {
@@ -31,5 +25,5 @@ XPCOMUtils.defineLazyGetter(lazy, "sharedData", () => {
  *  @return {Map.<string, Array<SessionDataItem>>}
  *     Map of session id to arrays of SessionDataItems.
  */
-const readSessionData = () =>
+export const readSessionData = () =>
   lazy.sharedData.get(lazy.SESSION_DATA_SHARED_DATA_KEY) || new Map();
diff --git a/remote/shared/messagehandler/test/browser/broadcast/head.js b/remote/shared/messagehandler/test/browser/broadcast/head.js
index 7c52eb4782ec..7bbe96ae97d5 100644
--- a/remote/shared/messagehandler/test/browser/broadcast/head.js
+++ b/remote/shared/messagehandler/test/browser/broadcast/head.js
@@ -36,8 +36,8 @@ function sendTestBroadcastCommand(
   contextDescriptor,
   rootMessageHandler
 ) {
-  const { WindowGlobalMessageHandler } = ChromeUtils.import(
-    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm"
+  const { WindowGlobalMessageHandler } = ChromeUtils.importESModule(
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs"
   );
 
   info("Send a test broadcast command");
diff --git a/remote/shared/messagehandler/test/browser/browser_events_dispatcher.js b/remote/shared/messagehandler/test/browser/browser_events_dispatcher.js
index 9770a7c549e3..32b9538c83db 100644
--- a/remote/shared/messagehandler/test/browser/browser_events_dispatcher.js
+++ b/remote/shared/messagehandler/test/browser/browser_events_dispatcher.js
@@ -3,8 +3,8 @@
 
 "use strict";
 
-const { WindowGlobalMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm"
+const { WindowGlobalMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs"
 );
 
 /**
diff --git a/remote/shared/messagehandler/test/browser/browser_events_module.js b/remote/shared/messagehandler/test/browser/browser_events_module.js
index 15ac138041c1..4032559e9ac9 100644
--- a/remote/shared/messagehandler/test/browser/browser_events_module.js
+++ b/remote/shared/messagehandler/test/browser/browser_events_module.js
@@ -3,14 +3,14 @@
 
 "use strict";
 
-const { RootMessageHandlerRegistry } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.jsm"
+const { RootMessageHandlerRegistry } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs"
 );
-const { RootMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm"
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
 );
-const { WindowGlobalMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm"
+const { WindowGlobalMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs"
 );
 
 /**
diff --git a/remote/shared/messagehandler/test/browser/browser_frame_context_utils.js b/remote/shared/messagehandler/test/browser/browser_frame_context_utils.js
index ecf968cd4ec3..eec9ce048f5c 100644
--- a/remote/shared/messagehandler/test/browser/browser_frame_context_utils.js
+++ b/remote/shared/messagehandler/test/browser/browser_frame_context_utils.js
@@ -3,8 +3,8 @@
 
 "use strict";
 
-const { isBrowsingContextCompatible } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/transports/FrameContextUtils.jsm"
+const { isBrowsingContextCompatible } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/transports/FrameContextUtils.sys.mjs"
 );
 const TEST_COM_PAGE = "https://example.com/document-builder.sjs?html=com";
 const TEST_NET_PAGE = "https://example.net/document-builder.sjs?html=net";
@@ -81,8 +81,8 @@ async function checkBrowsingContextCompatible(browser, browserId, expected) {
     browser,
     [browserId, expected],
     (_browserId, _expected) => {
-      const FrameContextUtils = ChromeUtils.import(
-        "chrome://remote/content/shared/messagehandler/transports/FrameContextUtils.jsm"
+      const FrameContextUtils = ChromeUtils.importESModule(
+        "chrome://remote/content/shared/messagehandler/transports/FrameContextUtils.sys.mjs"
       );
       is(
         FrameContextUtils.isBrowsingContextCompatible(content.browsingContext, {
diff --git a/remote/shared/messagehandler/test/browser/browser_handle_command_errors.js b/remote/shared/messagehandler/test/browser/browser_handle_command_errors.js
index a689607e1b21..0a46255d892b 100644
--- a/remote/shared/messagehandler/test/browser/browser_handle_command_errors.js
+++ b/remote/shared/messagehandler/test/browser/browser_handle_command_errors.js
@@ -3,11 +3,11 @@
 
 "use strict";
 
-const { RootMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm"
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
 );
-const { WindowGlobalMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm"
+const { WindowGlobalMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs"
 );
 
 // Check that errors from WindowGlobal modules can be caught by the consumer
diff --git a/remote/shared/messagehandler/test/browser/browser_handle_command_retry.js b/remote/shared/messagehandler/test/browser/browser_handle_command_retry.js
index 222ebfa9b377..eade7f655a32 100644
--- a/remote/shared/messagehandler/test/browser/browser_handle_command_retry.js
+++ b/remote/shared/messagehandler/test/browser/browser_handle_command_retry.js
@@ -3,8 +3,8 @@
 
 "use strict";
 
-const { WindowGlobalMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm"
+const { WindowGlobalMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs"
 );
 
 // We are forcing the actors to shutdown while queries are unresolved.
diff --git a/remote/shared/messagehandler/test/browser/browser_handle_simple_command.js b/remote/shared/messagehandler/test/browser/browser_handle_simple_command.js
index 0bd0af751345..a5024e5cce2a 100644
--- a/remote/shared/messagehandler/test/browser/browser_handle_simple_command.js
+++ b/remote/shared/messagehandler/test/browser/browser_handle_simple_command.js
@@ -3,11 +3,11 @@
 
 "use strict";
 
-const { RootMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm"
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
 );
-const { WindowGlobalMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm"
+const { WindowGlobalMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs"
 );
 
 // Test calling methods only implemented in the root version of a module.
diff --git a/remote/shared/messagehandler/test/browser/browser_registry.js b/remote/shared/messagehandler/test/browser/browser_registry.js
index 1d1dac6a174d..271597526e7a 100644
--- a/remote/shared/messagehandler/test/browser/browser_registry.js
+++ b/remote/shared/messagehandler/test/browser/browser_registry.js
@@ -3,11 +3,11 @@
 
 "use strict";
 
-const { MessageHandlerRegistry } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.jsm"
+const { MessageHandlerRegistry } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.sys.mjs"
 );
-const { RootMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm"
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
 );
 
 add_task(async function test_messageHandlerRegistry_API() {
diff --git a/remote/shared/messagehandler/test/browser/browser_session_data.js b/remote/shared/messagehandler/test/browser/browser_session_data.js
index c0baceca26d3..4959879de70b 100644
--- a/remote/shared/messagehandler/test/browser/browser_session_data.js
+++ b/remote/shared/messagehandler/test/browser/browser_session_data.js
@@ -3,14 +3,14 @@
 
 "use strict";
 
-const { MessageHandlerRegistry } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.jsm"
+const { MessageHandlerRegistry } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.sys.mjs"
 );
-const { RootMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm"
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
 );
-const { SessionData } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.jsm"
+const { SessionData } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.sys.mjs"
 );
 
 const TEST_PAGE = "http://example.com/document-builder.sjs?html=tab";
@@ -246,8 +246,8 @@ function checkSessionDataItem(item, moduleName, category, contextType, value) {
 
 function getSessionDataFromContent() {
   return SpecialPowers.spawn(gBrowser.selectedBrowser, [], () => {
-    const { readSessionData } = ChromeUtils.import(
-      "chrome://remote/content/shared/messagehandler/sessiondata/SessionDataReader.jsm"
+    const { readSessionData } = ChromeUtils.importESModule(
+      "chrome://remote/content/shared/messagehandler/sessiondata/SessionDataReader.sys.mjs"
     );
     return readSessionData();
   });
diff --git a/remote/shared/messagehandler/test/browser/browser_session_data_broadcast.js b/remote/shared/messagehandler/test/browser/browser_session_data_broadcast.js
index c68173777fdd..ffbabb5ad73e 100644
--- a/remote/shared/messagehandler/test/browser/browser_session_data_broadcast.js
+++ b/remote/shared/messagehandler/test/browser/browser_session_data_broadcast.js
@@ -3,8 +3,8 @@
 
 "use strict";
 
-const { RootMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm"
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
 );
 
 const TEST_PAGE = "https://example.com/document-builder.sjs?html=tab";
diff --git a/remote/shared/messagehandler/test/browser/browser_session_data_constructor_race.js b/remote/shared/messagehandler/test/browser/browser_session_data_constructor_race.js
index 476a6b9a73e8..e71c638ca35c 100644
--- a/remote/shared/messagehandler/test/browser/browser_session_data_constructor_race.js
+++ b/remote/shared/messagehandler/test/browser/browser_session_data_constructor_race.js
@@ -3,8 +3,8 @@
 
 "use strict";
 
-const { WindowGlobalMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm"
+const { WindowGlobalMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs"
 );
 
 const TEST_PAGE = "https://example.com/document-builder.sjs?html=tab";
diff --git a/remote/shared/messagehandler/test/browser/head.js b/remote/shared/messagehandler/test/browser/head.js
index 86e9f3183669..825db7afef40 100644
--- a/remote/shared/messagehandler/test/browser/head.js
+++ b/remote/shared/messagehandler/test/browser/head.js
@@ -4,8 +4,8 @@
 
 "use strict";
 
-var { ContextDescriptorType } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/MessageHandler.jsm"
+var { ContextDescriptorType } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs"
 );
 
 var contextDescriptorAll = {
@@ -13,8 +13,8 @@ var contextDescriptorAll = {
 };
 
 function createRootMessageHandler(sessionId) {
-  const { RootMessageHandlerRegistry } = ChromeUtils.import(
-    "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.jsm"
+  const { RootMessageHandlerRegistry } = ChromeUtils.importESModule(
+    "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs"
   );
   return RootMessageHandlerRegistry.getOrCreateMessageHandler(sessionId);
 }
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.jsm b/remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.sys.mjs
similarity index 70%
rename from remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.sys.mjs
index 1fd158617e70..e5d9c387196c 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.sys.mjs
@@ -2,10 +2,6 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-var EXPORTED_SYMBOLS = ["getModuleClass"];
-
 /**
  * Retrieve the WebDriver BiDi module class matching the provided module name
  * and folder.
@@ -20,10 +16,11 @@ var EXPORTED_SYMBOLS = ["getModuleClass"];
  * @throws {Error}
  *     If the provided module folder is unexpected.
  **/
-const getModuleClass = function(moduleName, moduleFolder) {
-  const path = `chrome://mochitests/content/browser/remote/shared/messagehandler/test/browser/resources/modules/${moduleFolder}/${moduleName}.jsm`;
+export const getModuleClass = function(moduleName, moduleFolder) {
+  const root = `chrome://mochitests/content/browser/remote/shared/messagehandler/test/`;
+  const path = `${root}browser/resources/modules/${moduleFolder}/${moduleName}.sys.mjs`;
   try {
-    return ChromeUtils.import(path)[moduleName];
+    return ChromeUtils.importESModule(path)[moduleName];
   } catch (e) {
     if (e.result == Cr.NS_ERROR_FILE_NOT_FOUND) {
       return null;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/root/command.jsm b/remote/shared/messagehandler/test/browser/resources/modules/root/command.sys.mjs
similarity index 80%
rename from remote/shared/messagehandler/test/browser/resources/modules/root/command.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/root/command.sys.mjs
index 2073536ce447..1f240292616e 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/root/command.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/root/command.sys.mjs
@@ -2,21 +2,9 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["command"];
-
-const { ContextDescriptorType } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/MessageHandler.jsm"
-);
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
-);
-
-const { WindowGlobalMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm"
-);
+import { ContextDescriptorType } from "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs";
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
+import { WindowGlobalMessageHandler } from "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs";
 
 class CommandModule extends Module {
   destroy() {}
@@ -84,4 +72,4 @@ class CommandModule extends Module {
   }
 }
 
-const command = CommandModule;
+export const command = CommandModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/root/event.jsm b/remote/shared/messagehandler/test/browser/resources/modules/root/event.sys.mjs
similarity index 68%
rename from remote/shared/messagehandler/test/browser/resources/modules/root/event.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/root/event.sys.mjs
index ecc386aaaa23..e49437e80d16 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/root/event.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/root/event.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["event"];
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
-);
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
 
 class EventModule extends Module {
   destroy() {}
@@ -24,4 +18,4 @@ class EventModule extends Module {
   }
 }
 
-const event = EventModule;
+export const event = EventModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/root/invalid.jsm b/remote/shared/messagehandler/test/browser/resources/modules/root/invalid.sys.mjs
similarity index 100%
rename from remote/shared/messagehandler/test/browser/resources/modules/root/invalid.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/root/invalid.sys.mjs
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/root/rootOnly.jsm b/remote/shared/messagehandler/test/browser/resources/modules/root/rootOnly.sys.mjs
similarity index 76%
rename from remote/shared/messagehandler/test/browser/resources/modules/root/rootOnly.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/root/rootOnly.sys.mjs
index 826db7050254..38e9c7802117 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/root/rootOnly.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/root/rootOnly.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["rootOnly"];
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
-);
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
 
 class RootOnlyModule extends Module {
   #sessionDataReceived;
@@ -38,4 +32,4 @@ class RootOnlyModule extends Module {
   }
 }
 
-const rootOnly = RootOnlyModule;
+export const rootOnly = RootOnlyModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/command.jsm b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/command.sys.mjs
similarity index 77%
rename from remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/command.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/command.sys.mjs
index 131bba8fd1a9..f9a2e5d4eb27 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/command.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/command.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["command"];
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
-);
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
 
 class CommandModule extends Module {
   destroy() {}
@@ -31,4 +25,4 @@ class CommandModule extends Module {
   }
 }
 
-const command = CommandModule;
+export const command = CommandModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.jsm b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.sys.mjs
similarity index 79%
rename from remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.sys.mjs
index 310f6b9d863b..2969e0a3e873 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal-in-root/event.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["event"];
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
-);
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
 
 class EventModule extends Module {
   destroy() {}
@@ -34,4 +28,4 @@ class EventModule extends Module {
   }
 }
 
-const event = EventModule;
+export const event = EventModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/command.jsm b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/command.sys.mjs
similarity index 92%
rename from remote/shared/messagehandler/test/browser/resources/modules/windowglobal/command.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/windowglobal/command.sys.mjs
index b69a3c8534e8..bca927dae914 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/command.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/command.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["command"];
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
-);
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
 
 class CommandModule extends Module {
   constructor(messageHandler) {
@@ -94,4 +88,4 @@ class CommandModule extends Module {
   }
 }
 
-const command = CommandModule;
+export const command = CommandModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/commandwindowglobalonly.jsm b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/commandwindowglobalonly.sys.mjs
similarity index 80%
rename from remote/shared/messagehandler/test/browser/resources/modules/windowglobal/commandwindowglobalonly.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/windowglobal/commandwindowglobalonly.sys.mjs
index 0daaf12694fe..1e4e6c15745a 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/commandwindowglobalonly.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/commandwindowglobalonly.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["commandwindowglobalonly"];
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
-);
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
 
 class CommandWindowGlobalOnlyModule extends Module {
   destroy() {}
@@ -44,4 +38,4 @@ class CommandWindowGlobalOnlyModule extends Module {
   }
 }
 
-const commandwindowglobalonly = CommandWindowGlobalOnlyModule;
+export const commandwindowglobalonly = CommandWindowGlobalOnlyModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.jsm b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.sys.mjs
similarity index 77%
rename from remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.sys.mjs
index 9a774a7ed264..5bb50cb83d89 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/event.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["event"];
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
-);
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
 
 class EventModule extends Module {
   destroy() {}
@@ -29,4 +23,4 @@ class EventModule extends Module {
   }
 }
 
-const event = EventModule;
+export const event = EventModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventemitter.jsm b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventemitter.sys.mjs
similarity index 88%
rename from remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventemitter.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventemitter.sys.mjs
index ad324e73c557..fcb8a7e44357 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventemitter.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventemitter.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["eventemitter"];
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
-);
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
 
 class EventEmitterModule extends Module {
   #isSubscribed;
@@ -69,4 +63,4 @@ class EventEmitterModule extends Module {
   }
 }
 
-const eventemitter = EventEmitterModule;
+export const eventemitter = EventEmitterModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventnointercept.jsm b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventnointercept.sys.mjs
similarity index 64%
rename from remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventnointercept.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventnointercept.sys.mjs
index ed8f53ed1d2a..48bbfbf95178 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventnointercept.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/eventnointercept.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["eventnointercept"];
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
-);
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
 
 class EventNoInterceptModule extends Module {
   destroy() {}
@@ -19,4 +13,4 @@ class EventNoInterceptModule extends Module {
   }
 }
 
-const eventnointercept = EventNoInterceptModule;
+export const eventnointercept = EventNoInterceptModule;
diff --git a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/retry.jsm b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/retry.sys.mjs
similarity index 93%
rename from remote/shared/messagehandler/test/browser/resources/modules/windowglobal/retry.jsm
rename to remote/shared/messagehandler/test/browser/resources/modules/windowglobal/retry.sys.mjs
index 845b47d6d984..f7b2279018d7 100644
--- a/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/retry.jsm
+++ b/remote/shared/messagehandler/test/browser/resources/modules/windowglobal/retry.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["retry"];
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
-);
+import { Module } from "chrome://remote/content/shared/messagehandler/Module.sys.mjs";
 
 // Store counters in the JSM scope to persist them across reloads.
 let callsToBlockedOneTime = 0;
@@ -87,4 +81,4 @@ class RetryModule extends Module {
   }
 }
 
-const retry = RetryModule;
+export const retry = RetryModule;
diff --git a/remote/shared/messagehandler/test/browser/webdriver/browser_session_execute_command_errors.js b/remote/shared/messagehandler/test/browser/webdriver/browser_session_execute_command_errors.js
index 8cf91359ef34..36a510bb29dc 100644
--- a/remote/shared/messagehandler/test/browser/webdriver/browser_session_execute_command_errors.js
+++ b/remote/shared/messagehandler/test/browser/webdriver/browser_session_execute_command_errors.js
@@ -3,12 +3,12 @@
 
 "use strict";
 
-const { WebDriverSession } = ChromeUtils.import(
-  "chrome://remote/content/shared/webdriver/Session.jsm"
+const { WebDriverSession } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Session.sys.mjs"
 );
 
-const { error } = ChromeUtils.import(
-  "chrome://remote/content/shared/webdriver/Errors.jsm"
+const { error } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Errors.sys.mjs"
 );
 
 add_task(async function test_execute_missing_command_error() {
diff --git a/remote/shared/messagehandler/test/xpcshell/test_Errors.js b/remote/shared/messagehandler/test/xpcshell/test_Errors.js
index afa40e82956f..33cfb5cfd78a 100644
--- a/remote/shared/messagehandler/test/xpcshell/test_Errors.js
+++ b/remote/shared/messagehandler/test/xpcshell/test_Errors.js
@@ -2,8 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-const { error } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Errors.jsm"
+const { error } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/Errors.sys.mjs"
 );
 
 // Note: this test file is similar to remote/shared/webdriver/test/xpcshell/test_Errors.js
diff --git a/remote/shared/messagehandler/test/xpcshell/test_SessionData.js b/remote/shared/messagehandler/test/xpcshell/test_SessionData.js
index faf7ab222d68..dfafffe1cd82 100644
--- a/remote/shared/messagehandler/test/xpcshell/test_SessionData.js
+++ b/remote/shared/messagehandler/test/xpcshell/test_SessionData.js
@@ -2,14 +2,14 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-const { ContextDescriptorType } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/MessageHandler.jsm"
+const { ContextDescriptorType } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs"
 );
-const { RootMessageHandler } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm"
+const { RootMessageHandler } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs"
 );
-const { SessionData } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.jsm"
+const { SessionData } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.sys.mjs"
 );
 
 add_task(async function test_sessionData() {
diff --git a/remote/shared/messagehandler/transports/FrameContextUtils.jsm b/remote/shared/messagehandler/transports/FrameContextUtils.sys.mjs
similarity index 93%
rename from remote/shared/messagehandler/transports/FrameContextUtils.jsm
rename to remote/shared/messagehandler/transports/FrameContextUtils.sys.mjs
index 1258f4a93713..341e7918ee4f 100644
--- a/remote/shared/messagehandler/transports/FrameContextUtils.jsm
+++ b/remote/shared/messagehandler/transports/FrameContextUtils.sys.mjs
@@ -2,10 +2,6 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["isBrowsingContextCompatible"];
-
 function isExtensionContext(browsingContext) {
   let principal;
   if (CanonicalBrowsingContext.isInstance(browsingContext)) {
@@ -43,7 +39,7 @@ function isParentProcess(browsingContext) {
  * @return {Boolean}
  *     True if the browsing context is valid, false otherwise.
  */
-function isBrowsingContextCompatible(browsingContext, options = {}) {
+export function isBrowsingContextCompatible(browsingContext, options = {}) {
   const { browserId } = options;
 
   // If a browserId was provided, skip browsing contexts which are not
diff --git a/remote/shared/messagehandler/transports/FrameTransport.jsm b/remote/shared/messagehandler/transports/FrameTransport.sys.mjs
similarity index 93%
rename from remote/shared/messagehandler/transports/FrameTransport.jsm
rename to remote/shared/messagehandler/transports/FrameTransport.sys.mjs
index 4f6f15f30a07..e2d48e00f4c2 100644
--- a/remote/shared/messagehandler/transports/FrameTransport.jsm
+++ b/remote/shared/messagehandler/transports/FrameTransport.sys.mjs
@@ -2,25 +2,19 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["FrameTransport"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
+ChromeUtils.defineESModuleGetters(lazy, {
   ContextDescriptorType:
-    "chrome://remote/content/shared/messagehandler/MessageHandler.jsm",
+    "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs",
   isBrowsingContextCompatible:
-    "chrome://remote/content/shared/messagehandler/transports/FrameContextUtils.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
+    "chrome://remote/content/shared/messagehandler/transports/FrameContextUtils.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
   MessageHandlerFrameActor:
-    "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.jsm",
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
+    "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
@@ -32,7 +26,7 @@ const MAX_RETRY_ATTEMPTS = 10;
  * with WINDOW_GLOBAL MessageHandlers via the MessageHandlerFrame JSWindow
  * actors.
  */
-class FrameTransport {
+export class FrameTransport {
   /**
    * @param {MessageHandler}
    *     The MessageHandler instance which owns this FrameTransport instance.
diff --git a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.jsm b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.sys.mjs
similarity index 74%
rename from remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.jsm
rename to remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.sys.mjs
index 4df324d42630..e537973fa54b 100644
--- a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.jsm
+++ b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.sys.mjs
@@ -2,31 +2,27 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-var EXPORTED_SYMBOLS = ["MessageHandlerFrameActor"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   ActorManagerParent: "resource://gre/modules/ActorManagerParent.jsm",
-
-  Log: "chrome://remote/content/shared/Log.jsm",
 });
 XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
 
 const FRAME_ACTOR_CONFIG = {
   parent: {
-    moduleURI:
-      "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm",
+    esModuleURI:
+      "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.sys.mjs",
   },
   child: {
-    moduleURI:
-      "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm",
+    esModuleURI:
+      "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.sys.mjs",
     events: {
       DOMWindowCreated: {},
     },
@@ -39,7 +35,7 @@ const FRAME_ACTOR_CONFIG = {
  * MessageHandlerFrameActor exposes a simple registration helper to lazily
  * register MessageHandlerFrame JSWindow actors.
  */
-const MessageHandlerFrameActor = {
+export const MessageHandlerFrameActor = {
   registered: false,
 
   register() {
diff --git a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.sys.mjs
similarity index 87%
rename from remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm
rename to remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.sys.mjs
index f2c9c1a507a8..4f0430df1b44 100644
--- a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm
+++ b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.sys.mjs
@@ -2,23 +2,15 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-var EXPORTED_SYMBOLS = ["MessageHandlerFrameChild"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
+ChromeUtils.defineESModuleGetters(lazy, {
   isBrowsingContextCompatible:
-    "chrome://remote/content/shared/messagehandler/transports/FrameContextUtils.jsm",
+    "chrome://remote/content/shared/messagehandler/transports/FrameContextUtils.sys.mjs",
   MessageHandlerRegistry:
-    "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.jsm",
+    "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.sys.mjs",
   WindowGlobalMessageHandler:
-    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm",
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs",
 });
 
 /**
@@ -26,7 +18,7 @@ XPCOMUtils.defineLazyModuleGetters(lazy, {
  * MessageHandlerFrame actor is used by FrameTransport to communicate between
  * ROOT MessageHandlers and WINDOW_GLOBAL MessageHandlers.
  */
-class MessageHandlerFrameChild extends JSWindowActorChild {
+export class MessageHandlerFrameChild extends JSWindowActorChild {
   actorCreated() {
     this.type = lazy.WindowGlobalMessageHandler.type;
     this.context = this.manager.browsingContext;
diff --git a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.sys.mjs
similarity index 86%
rename from remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm
rename to remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.sys.mjs
index c23d184be95d..9524fb08a594 100644
--- a/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm
+++ b/remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.sys.mjs
@@ -2,27 +2,21 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-var EXPORTED_SYMBOLS = ["MessageHandlerFrameParent"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  error: "chrome://remote/content/shared/messagehandler/Errors.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/messagehandler/Errors.sys.mjs",
   RootMessageHandlerRegistry:
-    "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.jsm",
+    "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs",
   WindowGlobalMessageHandler:
-    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm",
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "WebDriverError", () => {
-  return ChromeUtils.import(
-    "chrome://remote/content/shared/webdriver/Errors.jsm"
+  return ChromeUtils.importESModule(
+    "chrome://remote/content/shared/webdriver/Errors.sys.mjs"
   ).error.WebDriverError;
 });
 
@@ -31,7 +25,7 @@ XPCOMUtils.defineLazyGetter(lazy, "WebDriverError", () => {
  * MessageHandlerFrame actor is used by FrameTransport to communicate between
  * ROOT MessageHandlers and WINDOW_GLOBAL MessageHandlers.
  */
-class MessageHandlerFrameParent extends JSWindowActorParent {
+export class MessageHandlerFrameParent extends JSWindowActorParent {
   async receiveMessage(message) {
     switch (message.name) {
       case "MessageHandlerFrameChild:messageHandlerEvent":
diff --git a/remote/shared/test/browser/browser_TabManager.js b/remote/shared/test/browser/browser_TabManager.js
index fd57de317bae..71e1c60e4bb0 100644
--- a/remote/shared/test/browser/browser_TabManager.js
+++ b/remote/shared/test/browser/browser_TabManager.js
@@ -3,8 +3,8 @@
 
 "use strict";
 
-const { TabManager } = ChromeUtils.import(
-  "chrome://remote/content/shared/TabManager.jsm"
+const { TabManager } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/TabManager.sys.mjs"
 );
 
 const FRAME_URL = "https://example.com/document-builder.sjs?html=frame";
diff --git a/remote/shared/test/xpcshell/test_AppInfo.js b/remote/shared/test/xpcshell/test_AppInfo.js
index 7fad050f539f..d97d0d0cbbce 100644
--- a/remote/shared/test/xpcshell/test_AppInfo.js
+++ b/remote/shared/test/xpcshell/test_AppInfo.js
@@ -4,8 +4,8 @@
 
 "use strict";
 
-const { AppInfo } = ChromeUtils.import(
-  "chrome://remote/content/shared/AppInfo.jsm"
+const { AppInfo } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/AppInfo.sys.mjs"
 );
 
 // Minimal xpcshell tests for AppInfo; Services.appinfo.* is not available
diff --git a/remote/shared/test/xpcshell/test_Format.js b/remote/shared/test/xpcshell/test_Format.js
index e08f40725d52..40c4afb60fc8 100644
--- a/remote/shared/test/xpcshell/test_Format.js
+++ b/remote/shared/test/xpcshell/test_Format.js
@@ -3,8 +3,8 @@
 
 "use strict";
 
-const { truncate, pprint } = ChromeUtils.import(
-  "chrome://remote/content/shared/Format.jsm"
+const { truncate, pprint } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/Format.sys.mjs"
 );
 
 const MAX_STRING_LENGTH = 250;
diff --git a/remote/shared/test/xpcshell/test_Navigate.js b/remote/shared/test/xpcshell/test_Navigate.js
index 984eaf5ad981..c042c4ad40c2 100644
--- a/remote/shared/test/xpcshell/test_Navigate.js
+++ b/remote/shared/test/xpcshell/test_Navigate.js
@@ -7,7 +7,9 @@ const { setTimeout } = ChromeUtils.import("resource://gre/modules/Timer.jsm");
 const {
   ProgressListener,
   waitForInitialNavigationCompleted,
-} = ChromeUtils.import("chrome://remote/content/shared/Navigate.jsm");
+} = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/Navigate.sys.mjs"
+);
 
 const CURRENT_URI = Services.io.newURI("http://foo.bar/");
 const INITIAL_URI = Services.io.newURI("about:blank");
diff --git a/remote/shared/test/xpcshell/test_RecommendedPreferences.js b/remote/shared/test/xpcshell/test_RecommendedPreferences.js
index 05cbfdcdbe0a..d42482d1d82b 100644
--- a/remote/shared/test/xpcshell/test_RecommendedPreferences.js
+++ b/remote/shared/test/xpcshell/test_RecommendedPreferences.js
@@ -2,8 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-const { RecommendedPreferences } = ChromeUtils.import(
-  "chrome://remote/content/shared/RecommendedPreferences.jsm"
+const { RecommendedPreferences } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/RecommendedPreferences.sys.mjs"
 );
 
 const COMMON_PREF = "toolkit.startup.max_resumed_crashes";
diff --git a/remote/shared/test/xpcshell/test_Stack.js b/remote/shared/test/xpcshell/test_Stack.js
index 7872821856a1..c41c5f0240fb 100644
--- a/remote/shared/test/xpcshell/test_Stack.js
+++ b/remote/shared/test/xpcshell/test_Stack.js
@@ -2,8 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-const { getFramesFromStack, isChromeFrame } = ChromeUtils.import(
-  "chrome://remote/content/shared/Stack.jsm"
+const { getFramesFromStack, isChromeFrame } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/Stack.sys.mjs"
 );
 
 const sourceFrames = [
diff --git a/remote/shared/test/xpcshell/test_Sync.js b/remote/shared/test/xpcshell/test_Sync.js
index 163c60ac4579..83438ce8c406 100644
--- a/remote/shared/test/xpcshell/test_Sync.js
+++ b/remote/shared/test/xpcshell/test_Sync.js
@@ -9,7 +9,7 @@ const {
   Deferred,
   EventPromise,
   PollPromise,
-} = ChromeUtils.import("chrome://remote/content/shared/Sync.jsm");
+} = ChromeUtils.importESModule("chrome://remote/content/shared/Sync.sys.mjs");
 
 /**
  * Mimic a DOM node for listening for events.
@@ -272,7 +272,9 @@ add_task(async function test_EventPromise_wantUntrustedEvent() {
 add_task(function test_executeSoon_callback() {
   // executeSoon() is already defined for xpcshell in head.js. As such import
   // our implementation into a custom namespace.
-  let sync = ChromeUtils.import("chrome://remote/content/shared/Sync.jsm");
+  let sync = ChromeUtils.importESModule(
+    "chrome://remote/content/shared/Sync.sys.mjs"
+  );
 
   for (let func of ["foo", null, true, [], {}]) {
     Assert.throws(() => sync.executeSoon(func), /TypeError/);
diff --git a/remote/shared/test/xpcshell/test_TabManager.js b/remote/shared/test/xpcshell/test_TabManager.js
index 0806b1bf481d..c0676531baf1 100644
--- a/remote/shared/test/xpcshell/test_TabManager.js
+++ b/remote/shared/test/xpcshell/test_TabManager.js
@@ -2,8 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-const { TabManager } = ChromeUtils.import(
-  "chrome://remote/content/shared/TabManager.jsm"
+const { TabManager } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/TabManager.sys.mjs"
 );
 
 class MockTopBrowsingContext {
diff --git a/remote/shared/webdriver/Assert.jsm b/remote/shared/webdriver/Assert.sys.mjs
similarity index 96%
rename from remote/shared/webdriver/Assert.jsm
rename to remote/shared/webdriver/Assert.sys.mjs
index 86018cd2e2b8..ed0278308a19 100644
--- a/remote/shared/webdriver/Assert.jsm
+++ b/remote/shared/webdriver/Assert.sys.mjs
@@ -2,20 +2,12 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["assert"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  AppInfo: "chrome://remote/content/shared/AppInfo.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  pprint: "chrome://remote/content/shared/Format.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
 });
 
 /**
@@ -23,7 +15,7 @@ XPCOMUtils.defineLazyModuleGetters(lazy, {
  *
  * @namespace
  */
-const assert = {};
+export const assert = {};
 
 /**
  * Asserts that WebDriver has an active session.
diff --git a/remote/shared/webdriver/Capabilities.jsm b/remote/shared/webdriver/Capabilities.sys.mjs
similarity index 96%
rename from remote/shared/webdriver/Capabilities.jsm
rename to remote/shared/webdriver/Capabilities.sys.mjs
index eae377bbe23f..3fc4fb888133 100644
--- a/remote/shared/webdriver/Capabilities.jsm
+++ b/remote/shared/webdriver/Capabilities.sys.mjs
@@ -2,29 +2,20 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = [
-  "Capabilities",
-  "PageLoadStrategy",
-  "Proxy",
-  "Timeouts",
-  "UnhandledPromptBehavior",
-];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   Preferences: "resource://gre/modules/Preferences.jsm",
 
-  AppInfo: "chrome://remote/content/shared/AppInfo.jsm",
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  pprint: "chrome://remote/content/shared/Format.jsm",
   RemoteAgent: "chrome://remote/content/components/RemoteAgent.jsm",
 });
 
@@ -33,7 +24,7 @@ XPCOMUtils.defineLazyGetter(lazy, "remoteAgent", () => {
 });
 
 /** Representation of WebDriver session timeouts. */
-class Timeouts {
+export class Timeouts {
   constructor() {
     // disabled
     this.implicit = 0;
@@ -105,7 +96,7 @@ class Timeouts {
  *
  * @enum
  */
-const PageLoadStrategy = {
+export const PageLoadStrategy = {
   /** No page load strategy.  Navigation will return immediately. */
   None: "none",
   /**
@@ -121,7 +112,7 @@ const PageLoadStrategy = {
 };
 
 /** Proxy configuration object representation. */
-class Proxy {
+export class Proxy {
   /** @class */
   constructor() {
     this.proxyType = null;
@@ -407,7 +398,7 @@ class Proxy {
  *
  * @enum
  */
-const UnhandledPromptBehavior = {
+export const UnhandledPromptBehavior = {
   /** All simple dialogs encountered should be accepted. */
   Accept: "accept",
   /**
@@ -427,7 +418,7 @@ const UnhandledPromptBehavior = {
 };
 
 /** WebDriver session capabilities representation. */
-class Capabilities extends Map {
+export class Capabilities extends Map {
   /** @class */
   constructor() {
     super([
diff --git a/remote/shared/webdriver/Errors.jsm b/remote/shared/webdriver/Errors.sys.mjs
similarity index 97%
rename from remote/shared/webdriver/Errors.jsm
rename to remote/shared/webdriver/Errors.sys.mjs
index 57b7e3bbe999..c1bd46588d8c 100644
--- a/remote/shared/webdriver/Errors.jsm
+++ b/remote/shared/webdriver/Errors.sys.mjs
@@ -2,22 +2,12 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["error"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
-const { RemoteError } = ChromeUtils.import(
-  "chrome://remote/content/shared/RemoteError.jsm"
-);
+import { RemoteError } from "chrome://remote/content/shared/RemoteError.sys.mjs";
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  pprint: "chrome://remote/content/shared/Format.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  pprint: "chrome://remote/content/shared/Format.sys.mjs",
 });
 
 const ERRORS = new Set([
@@ -62,7 +52,7 @@ const BUILTIN_ERRORS = new Set([
 ]);
 
 /** @namespace */
-const error = {
+export const error = {
   /**
    * Check if ``val`` is an instance of the ``Error`` prototype.
    *
diff --git a/remote/shared/webdriver/KeyData.jsm b/remote/shared/webdriver/KeyData.sys.mjs
similarity index 98%
rename from remote/shared/webdriver/KeyData.jsm
rename to remote/shared/webdriver/KeyData.sys.mjs
index 732aa9faf29d..5d2570686fe3 100644
--- a/remote/shared/webdriver/KeyData.jsm
+++ b/remote/shared/webdriver/KeyData.sys.mjs
@@ -2,13 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["keyData"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const KEY_DATA = {
   " ": { code: "Space" },
@@ -313,7 +307,7 @@ XPCOMUtils.defineLazyGetter(lazy, "SHIFT_DATA", () => {
   return shiftData;
 });
 
-const keyData = {
+export const keyData = {
   /**
    * Get key event data for a given key character.
    *
diff --git a/remote/shared/webdriver/Session.jsm b/remote/shared/webdriver/Session.sys.mjs
similarity index 96%
rename from remote/shared/webdriver/Session.jsm
rename to remote/shared/webdriver/Session.sys.mjs
index 130313f9936c..f9f5a92ab6db 100644
--- a/remote/shared/webdriver/Session.jsm
+++ b/remote/shared/webdriver/Session.sys.mjs
@@ -2,26 +2,23 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-"use strict";
-
-const EXPORTED_SYMBOLS = ["WebDriverSession"];
-
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  Capabilities: "chrome://remote/content/shared/webdriver/Capabilities.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  RootMessageHandler:
+    "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs",
+  RootMessageHandlerRegistry:
+    "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   accessibility: "chrome://remote/content/marionette/accessibility.js",
   allowAllCerts: "chrome://remote/content/marionette/cert.js",
-  Capabilities: "chrome://remote/content/shared/webdriver/Capabilities.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
-  RootMessageHandler:
-    "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm",
-  RootMessageHandlerRegistry:
-    "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.jsm",
   WebDriverBiDiConnection:
     "chrome://remote/content/webdriver-bidi/WebDriverBiDiConnection.jsm",
   WebSocketHandshake: "chrome://remote/content/server/WebSocketHandshake.jsm",
@@ -32,7 +29,7 @@ XPCOMUtils.defineLazyGetter(lazy, "logger", () => lazy.Log.get());
 /**
  * Representation of WebDriver session.
  */
-class WebDriverSession {
+export class WebDriverSession {
   /**
    * Construct a new WebDriver session.
    *
diff --git a/remote/shared/webdriver/test/xpcshell/test_Assert.js b/remote/shared/webdriver/test/xpcshell/test_Assert.js
index 47bff04f77e5..a16ff77fb000 100644
--- a/remote/shared/webdriver/test/xpcshell/test_Assert.js
+++ b/remote/shared/webdriver/test/xpcshell/test_Assert.js
@@ -5,11 +5,11 @@
 "use strict";
 /* eslint-disable no-array-constructor, no-new-object */
 
-const { assert } = ChromeUtils.import(
-  "chrome://remote/content/shared/webdriver/Assert.jsm"
+const { assert } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Assert.sys.mjs"
 );
-const { error } = ChromeUtils.import(
-  "chrome://remote/content/shared/webdriver/Errors.jsm"
+const { error } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Errors.sys.mjs"
 );
 
 add_test(function test_session() {
diff --git a/remote/shared/webdriver/test/xpcshell/test_Capabilities.js b/remote/shared/webdriver/test/xpcshell/test_Capabilities.js
index 8438bfd47443..6d08f87380d1 100644
--- a/remote/shared/webdriver/test/xpcshell/test_Capabilities.js
+++ b/remote/shared/webdriver/test/xpcshell/test_Capabilities.js
@@ -8,11 +8,11 @@ const { Preferences } = ChromeUtils.import(
   "resource://gre/modules/Preferences.jsm"
 );
 
-const { AppInfo } = ChromeUtils.import(
-  "chrome://remote/content/shared/AppInfo.jsm"
+const { AppInfo } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/AppInfo.sys.mjs"
 );
-const { error } = ChromeUtils.import(
-  "chrome://remote/content/shared/webdriver/Errors.jsm"
+const { error } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Errors.sys.mjs"
 );
 const {
   Capabilities,
@@ -20,8 +20,8 @@ const {
   Proxy,
   Timeouts,
   UnhandledPromptBehavior,
-} = ChromeUtils.import(
-  "chrome://remote/content/shared/webdriver/Capabilities.jsm"
+} = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Capabilities.sys.mjs"
 );
 
 add_test(function test_Timeouts_ctor() {
diff --git a/remote/shared/webdriver/test/xpcshell/test_Errors.js b/remote/shared/webdriver/test/xpcshell/test_Errors.js
index c968a037d5b0..1510198afe4b 100644
--- a/remote/shared/webdriver/test/xpcshell/test_Errors.js
+++ b/remote/shared/webdriver/test/xpcshell/test_Errors.js
@@ -2,8 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-const { error } = ChromeUtils.import(
-  "chrome://remote/content/shared/webdriver/Errors.jsm"
+const { error } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Errors.sys.mjs"
 );
 
 function notok(condition) {
diff --git a/remote/shared/webdriver/test/xpcshell/test_Session.js b/remote/shared/webdriver/test/xpcshell/test_Session.js
index 9ce6b1b22bc9..76ab59f82af2 100644
--- a/remote/shared/webdriver/test/xpcshell/test_Session.js
+++ b/remote/shared/webdriver/test/xpcshell/test_Session.js
@@ -4,11 +4,11 @@
 
 "use strict";
 
-const { Capabilities, Timeouts } = ChromeUtils.import(
-  "chrome://remote/content/shared/webdriver/Capabilities.jsm"
+const { Capabilities, Timeouts } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Capabilities.sys.mjs"
 );
-const { WebDriverSession } = ChromeUtils.import(
-  "chrome://remote/content/shared/webdriver/Session.jsm"
+const { WebDriverSession } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/webdriver/Session.sys.mjs"
 );
 
 add_test(function test_WebDriverSession_ctor() {
diff --git a/remote/webdriver-bidi/RemoteValue.jsm b/remote/webdriver-bidi/RemoteValue.jsm
index e85946e24066..d8f86e4ffe9c 100644
--- a/remote/webdriver-bidi/RemoteValue.jsm
+++ b/remote/webdriver-bidi/RemoteValue.jsm
@@ -17,10 +17,10 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/webdriver-bidi/WebDriverBiDi.jsm b/remote/webdriver-bidi/WebDriverBiDi.jsm
index af258e76da92..785c461b69cf 100644
--- a/remote/webdriver-bidi/WebDriverBiDi.jsm
+++ b/remote/webdriver-bidi/WebDriverBiDi.jsm
@@ -12,12 +12,15 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  WebDriverSession: "chrome://remote/content/shared/webdriver/Session.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
   WebDriverNewSessionHandler:
     "chrome://remote/content/webdriver-bidi/NewSessionHandler.jsm",
-  WebDriverSession: "chrome://remote/content/shared/webdriver/Session.jsm",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/webdriver-bidi/WebDriverBiDiConnection.jsm b/remote/webdriver-bidi/WebDriverBiDiConnection.jsm
index 4b7d7a25336d..598fce2c0279 100644
--- a/remote/webdriver-bidi/WebDriverBiDiConnection.jsm
+++ b/remote/webdriver-bidi/WebDriverBiDiConnection.jsm
@@ -10,16 +10,19 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
   "resource://gre/modules/XPCOMUtils.sys.mjs"
 );
 
-const { WebSocketConnection } = ChromeUtils.import(
-  "chrome://remote/content/shared/WebSocketConnection.jsm"
+const { WebSocketConnection } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/WebSocketConnection.sys.mjs"
 );
 
 const lazy = {};
 
+ChromeUtils.defineESModuleGetters(lazy, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
   RemoteAgent: "chrome://remote/content/components/RemoteAgent.jsm",
 });
 
diff --git a/remote/webdriver-bidi/modules/root/browsingContext.jsm b/remote/webdriver-bidi/modules/root/browsingContext.jsm
index efc34534173d..b1aea70b1fa5 100644
--- a/remote/webdriver-bidi/modules/root/browsingContext.jsm
+++ b/remote/webdriver-bidi/modules/root/browsingContext.jsm
@@ -10,28 +10,28 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
   "resource://gre/modules/XPCOMUtils.sys.mjs"
 );
 
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
+const { Module } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/Module.sys.mjs"
 );
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  AppInfo: "chrome://remote/content/shared/AppInfo.jsm",
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  AppInfo: "chrome://remote/content/shared/AppInfo.sys.mjs",
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
   BrowsingContextListener:
-    "chrome://remote/content/shared/listeners/BrowsingContextListener.jsm",
+    "chrome://remote/content/shared/listeners/BrowsingContextListener.sys.mjs",
   ContextDescriptorType:
-    "chrome://remote/content/shared/messagehandler/MessageHandler.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  Log: "chrome://remote/content/shared/Log.jsm",
-  ProgressListener: "chrome://remote/content/shared/Navigate.jsm",
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
+    "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  Log: "chrome://remote/content/shared/Log.sys.mjs",
+  ProgressListener: "chrome://remote/content/shared/Navigate.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
   waitForInitialNavigationCompleted:
-    "chrome://remote/content/shared/Navigate.jsm",
+    "chrome://remote/content/shared/Navigate.sys.mjs",
   WindowGlobalMessageHandler:
-    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm",
-  windowManager: "chrome://remote/content/shared/WindowManager.jsm",
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs",
+  windowManager: "chrome://remote/content/shared/WindowManager.sys.mjs",
 });
 
 XPCOMUtils.defineLazyGetter(lazy, "logger", () =>
diff --git a/remote/webdriver-bidi/modules/root/log.jsm b/remote/webdriver-bidi/modules/root/log.jsm
index 8dde84bae730..82094ca05388 100644
--- a/remote/webdriver-bidi/modules/root/log.jsm
+++ b/remote/webdriver-bidi/modules/root/log.jsm
@@ -6,8 +6,8 @@
 
 const EXPORTED_SYMBOLS = ["log"];
 
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
+const { Module } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/Module.sys.mjs"
 );
 
 class LogModule extends Module {
diff --git a/remote/webdriver-bidi/modules/root/script.jsm b/remote/webdriver-bidi/modules/root/script.jsm
index f3332d73089d..4f41ccb14f7e 100644
--- a/remote/webdriver-bidi/modules/root/script.jsm
+++ b/remote/webdriver-bidi/modules/root/script.jsm
@@ -10,22 +10,25 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
   "resource://gre/modules/XPCOMUtils.sys.mjs"
 );
 
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
+const { Module } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/Module.sys.mjs"
 );
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
   ContextDescriptorType:
-    "chrome://remote/content/shared/messagehandler/MessageHandler.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
+    "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
+  WindowGlobalMessageHandler:
+    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.sys.mjs",
+});
+
+XPCOMUtils.defineLazyModuleGetters(lazy, {
   OwnershipModel: "chrome://remote/content/webdriver-bidi/RemoteValue.jsm",
   RealmType: "chrome://remote/content/webdriver-bidi/Realm.jsm",
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
-  WindowGlobalMessageHandler:
-    "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm",
 });
 
 class ScriptModule extends Module {
diff --git a/remote/webdriver-bidi/modules/root/session.jsm b/remote/webdriver-bidi/modules/root/session.jsm
index b96c06532861..99344604641f 100644
--- a/remote/webdriver-bidi/modules/root/session.jsm
+++ b/remote/webdriver-bidi/modules/root/session.jsm
@@ -6,23 +6,19 @@
 
 const EXPORTED_SYMBOLS = ["session"];
 
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
+const { Module } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/Module.sys.mjs"
 );
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  assert: "chrome://remote/content/shared/webdriver/Assert.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  assert: "chrome://remote/content/shared/webdriver/Assert.sys.mjs",
   ContextDescriptorType:
-    "chrome://remote/content/shared/messagehandler/MessageHandler.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
+    "chrome://remote/content/shared/messagehandler/MessageHandler.sys.mjs",
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
   RootMessageHandler:
-    "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm",
+    "chrome://remote/content/shared/messagehandler/RootMessageHandler.sys.mjs",
 });
 
 class SessionModule extends Module {
diff --git a/remote/webdriver-bidi/modules/windowglobal-in-root/browsingContext.jsm b/remote/webdriver-bidi/modules/windowglobal-in-root/browsingContext.jsm
index ce110e1e1bf2..dedea7470705 100644
--- a/remote/webdriver-bidi/modules/windowglobal-in-root/browsingContext.jsm
+++ b/remote/webdriver-bidi/modules/windowglobal-in-root/browsingContext.jsm
@@ -6,18 +6,14 @@
 
 const EXPORTED_SYMBOLS = ["browsingContext"];
 
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
+const { Module } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/Module.sys.mjs"
 );
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
 });
 
 class BrowsingContextModule extends Module {
diff --git a/remote/webdriver-bidi/modules/windowglobal-in-root/log.jsm b/remote/webdriver-bidi/modules/windowglobal-in-root/log.jsm
index 9db9ac96f8b7..abe767c71a2b 100644
--- a/remote/webdriver-bidi/modules/windowglobal-in-root/log.jsm
+++ b/remote/webdriver-bidi/modules/windowglobal-in-root/log.jsm
@@ -6,18 +6,14 @@
 
 const EXPORTED_SYMBOLS = ["log"];
 
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
+const { Module } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/Module.sys.mjs"
 );
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  TabManager: "chrome://remote/content/shared/TabManager.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  TabManager: "chrome://remote/content/shared/TabManager.sys.mjs",
 });
 
 class LogModule extends Module {
diff --git a/remote/webdriver-bidi/modules/windowglobal/browsingContext.jsm b/remote/webdriver-bidi/modules/windowglobal/browsingContext.jsm
index 3b6db0d06bb8..94feb57d4298 100644
--- a/remote/webdriver-bidi/modules/windowglobal/browsingContext.jsm
+++ b/remote/webdriver-bidi/modules/windowglobal/browsingContext.jsm
@@ -6,18 +6,14 @@
 
 const EXPORTED_SYMBOLS = ["browsingContext"];
 
-const { XPCOMUtils } = ChromeUtils.importESModule(
-  "resource://gre/modules/XPCOMUtils.sys.mjs"
-);
-
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
+const { Module } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/Module.sys.mjs"
 );
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
-  LoadListener: "chrome://remote/content/shared/listeners/LoadListener.jsm",
+ChromeUtils.defineESModuleGetters(lazy, {
+  LoadListener: "chrome://remote/content/shared/listeners/LoadListener.sys.mjs",
 });
 
 class BrowsingContextModule extends Module {
diff --git a/remote/webdriver-bidi/modules/windowglobal/log.jsm b/remote/webdriver-bidi/modules/windowglobal/log.jsm
index f95ac4d8953d..7cef98ff5c34 100644
--- a/remote/webdriver-bidi/modules/windowglobal/log.jsm
+++ b/remote/webdriver-bidi/modules/windowglobal/log.jsm
@@ -10,18 +10,21 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
   "resource://gre/modules/XPCOMUtils.sys.mjs"
 );
 
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
+const { Module } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/Module.sys.mjs"
 );
 
 const lazy = {};
 
-XPCOMUtils.defineLazyModuleGetters(lazy, {
+ChromeUtils.defineESModuleGetters(lazy, {
   ConsoleAPIListener:
-    "chrome://remote/content/shared/listeners/ConsoleAPIListener.jsm",
+    "chrome://remote/content/shared/listeners/ConsoleAPIListener.sys.mjs",
   ConsoleListener:
-    "chrome://remote/content/shared/listeners/ConsoleListener.jsm",
-  isChromeFrame: "chrome://remote/content/shared/Stack.jsm",
+    "chrome://remote/content/shared/listeners/ConsoleListener.sys.mjs",
+  isChromeFrame: "chrome://remote/content/shared/Stack.sys.mjs",
+});
+
+XPCOMUtils.defineLazyModuleGetters(lazy, {
   OwnershipModel: "chrome://remote/content/webdriver-bidi/RemoteValue.jsm",
   serialize: "chrome://remote/content/webdriver-bidi/RemoteValue.jsm",
 });
diff --git a/remote/webdriver-bidi/modules/windowglobal/script.jsm b/remote/webdriver-bidi/modules/windowglobal/script.jsm
index 4ec15043fbdc..2be6d1547e3d 100644
--- a/remote/webdriver-bidi/modules/windowglobal/script.jsm
+++ b/remote/webdriver-bidi/modules/windowglobal/script.jsm
@@ -10,16 +10,20 @@ const { XPCOMUtils } = ChromeUtils.importESModule(
   "resource://gre/modules/XPCOMUtils.sys.mjs"
 );
 
-const { Module } = ChromeUtils.import(
-  "chrome://remote/content/shared/messagehandler/Module.jsm"
+const { Module } = ChromeUtils.importESModule(
+  "chrome://remote/content/shared/messagehandler/Module.sys.mjs"
 );
 
 const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  error: "chrome://remote/content/shared/webdriver/Errors.sys.mjs",
+  getFramesFromStack: "chrome://remote/content/shared/Stack.sys.mjs",
+  isChromeFrame: "chrome://remote/content/shared/Stack.sys.mjs",
+});
+
 XPCOMUtils.defineLazyModuleGetters(lazy, {
   deserialize: "chrome://remote/content/webdriver-bidi/RemoteValue.jsm",
-  error: "chrome://remote/content/shared/webdriver/Errors.jsm",
-  getFramesFromStack: "chrome://remote/content/shared/Stack.jsm",
-  isChromeFrame: "chrome://remote/content/shared/Stack.jsm",
   serialize: "chrome://remote/content/webdriver-bidi/RemoteValue.jsm",
   stringify: "chrome://remote/content/webdriver-bidi/RemoteValue.jsm",
   WindowRealm: "chrome://remote/content/webdriver-bidi/Realm.jsm",
diff --git a/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py b/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py
index c3bb6e3e7a8d..cb9b6f554145 100644
--- a/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py
+++ b/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py
@@ -96,8 +96,8 @@ class WindowManagerMixin(object):
             with self.marionette.using_context("chrome"):
                 return self.marionette.execute_script(
                     """
-                  const { windowManager } = ChromeUtils.import(
-                    "chrome://remote/content/shared/WindowManager.jsm"
+                  const { windowManager } = ChromeUtils.importESModule(
+                    "chrome://remote/content/shared/WindowManager.sys.mjs"
                   );
                   const win = windowManager.findWindowByHandle(arguments[0]).win;
                   return win.document.readyState == "complete";
diff --git a/tools/esmify/map.json b/tools/esmify/map.json
index edac898d5aba..6ea24e11bff8 100644
--- a/tools/esmify/map.json
+++ b/tools/esmify/map.json
@@ -21,8 +21,6 @@
     "accessible/tests/browser/Layout.jsm",
   "chrome://mochitests/content/browser/devtools/client/shared/sourceeditor/test/CodeMirrorTestActors.jsm":
     "devtools/client/shared/sourceeditor/test/CodeMirrorTestActors.jsm",
-  "chrome://mochitests/content/browser/remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.jsm":
-    "remote/shared/messagehandler/test/browser/resources/modules/ModuleRegistry.jsm",
   "chrome://mochitests/content/chrome/docshell/test/chrome/DocShellHelpers.jsm":
     "docshell/test/chrome/DocShellHelpers.jsm",
   "chrome://mochitests/content/chrome/dom/console/tests/console.jsm":
@@ -188,73 +186,6 @@
     "remote/server/WebSocketHandshake.jsm",
   "chrome://remote/content/server/WebSocketTransport.jsm":
     "remote/server/WebSocketTransport.jsm",
-  "chrome://remote/content/shared/AppInfo.jsm": "remote/shared/AppInfo.jsm",
-  "chrome://remote/content/shared/Format.jsm": "remote/shared/Format.jsm",
-  "chrome://remote/content/shared/Log.jsm": "remote/shared/Log.jsm",
-  "chrome://remote/content/shared/MobileTabBrowser.jsm":
-    "remote/shared/MobileTabBrowser.jsm",
-  "chrome://remote/content/shared/Navigate.jsm": "remote/shared/Navigate.jsm",
-  "chrome://remote/content/shared/PDF.jsm": "remote/shared/PDF.jsm",
-  "chrome://remote/content/shared/RecommendedPreferences.jsm":
-    "remote/shared/RecommendedPreferences.jsm",
-  "chrome://remote/content/shared/Stack.jsm": "remote/shared/Stack.jsm",
-  "chrome://remote/content/shared/Sync.jsm": "remote/shared/Sync.jsm",
-  "chrome://remote/content/shared/TabManager.jsm":
-    "remote/shared/TabManager.jsm",
-  "chrome://remote/content/shared/WebSocketConnection.jsm":
-    "remote/shared/WebSocketConnection.jsm",
-  "chrome://remote/content/shared/WindowManager.jsm":
-    "remote/shared/WindowManager.jsm",
-  "chrome://remote/content/shared/listeners/BrowsingContextListener.jsm":
-    "remote/shared/listeners/BrowsingContextListener.jsm",
-  "chrome://remote/content/shared/listeners/ConsoleAPIListener.jsm":
-    "remote/shared/listeners/ConsoleAPIListener.jsm",
-  "chrome://remote/content/shared/listeners/ConsoleListener.jsm":
-    "remote/shared/listeners/ConsoleListener.jsm",
-  "chrome://remote/content/shared/listeners/LoadListener.jsm":
-    "remote/shared/listeners/LoadListener.jsm",
-  "chrome://remote/content/shared/messagehandler/Errors.jsm":
-    "remote/shared/messagehandler/Errors.jsm",
-  "chrome://remote/content/shared/messagehandler/EventsDispatcher.jsm":
-    "remote/shared/messagehandler/EventsDispatcher.jsm",
-  "chrome://remote/content/shared/messagehandler/MessageHandler.jsm":
-    "remote/shared/messagehandler/MessageHandler.jsm",
-  "chrome://remote/content/shared/messagehandler/MessageHandlerRegistry.jsm":
-    "remote/shared/messagehandler/MessageHandlerRegistry.jsm",
-  "chrome://remote/content/shared/messagehandler/Module.jsm":
-    "remote/shared/messagehandler/Module.jsm",
-  "chrome://remote/content/shared/messagehandler/ModuleCache.jsm":
-    "remote/shared/messagehandler/ModuleCache.jsm",
-  "chrome://remote/content/shared/messagehandler/RootMessageHandler.jsm":
-    "remote/shared/messagehandler/RootMessageHandler.jsm",
-  "chrome://remote/content/shared/messagehandler/RootMessageHandlerRegistry.jsm":
-    "remote/shared/messagehandler/RootMessageHandlerRegistry.jsm",
-  "chrome://remote/content/shared/messagehandler/WindowGlobalMessageHandler.jsm":
-    "remote/shared/messagehandler/WindowGlobalMessageHandler.jsm",
-  "chrome://remote/content/shared/messagehandler/sessiondata/SessionData.jsm":
-    "remote/shared/messagehandler/sessiondata/SessionData.jsm",
-  "chrome://remote/content/shared/messagehandler/sessiondata/SessionDataReader.jsm":
-    "remote/shared/messagehandler/sessiondata/SessionDataReader.jsm",
-  "chrome://remote/content/shared/messagehandler/transports/FrameContextUtils.jsm":
-    "remote/shared/messagehandler/transports/FrameContextUtils.jsm",
-  "chrome://remote/content/shared/messagehandler/transports/FrameTransport.jsm":
-    "remote/shared/messagehandler/transports/FrameTransport.jsm",
-  "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.jsm":
-    "remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameActor.jsm",
-  "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm":
-    "remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameChild.jsm",
-  "chrome://remote/content/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm":
-    "remote/shared/messagehandler/transports/js-window-actors/MessageHandlerFrameParent.jsm",
-  "chrome://remote/content/shared/webdriver/Assert.jsm":
-    "remote/shared/webdriver/Assert.jsm",
-  "chrome://remote/content/shared/webdriver/Capabilities.jsm":
-    "remote/shared/webdriver/Capabilities.jsm",
-  "chrome://remote/content/shared/webdriver/Errors.jsm":
-    "remote/shared/webdriver/Errors.jsm",
-  "chrome://remote/content/shared/webdriver/KeyData.jsm":
-    "remote/shared/webdriver/KeyData.jsm",
-  "chrome://remote/content/shared/webdriver/Session.jsm":
-    "remote/shared/webdriver/Session.jsm",
   "chrome://remote/content/webdriver-bidi/NewSessionHandler.jsm":
     "remote/webdriver-bidi/NewSessionHandler.jsm",
   "chrome://remote/content/webdriver-bidi/RemoteValue.jsm":