diff --git a/browser/base/content/test/fullscreen/browser_fullscreen_permissions_prompt.js b/browser/base/content/test/fullscreen/browser_fullscreen_permissions_prompt.js
index 5f363fbac96b..26e4ec971a66 100644
--- a/browser/base/content/test/fullscreen/browser_fullscreen_permissions_prompt.js
+++ b/browser/base/content/test/fullscreen/browser_fullscreen_permissions_prompt.js
@@ -18,14 +18,11 @@ async function requestNotificationPermission(browser) {
 }
 
 async function requestCameraPermission(browser) {
-  return SpecialPowers.spawn(browser, [], () => {
-    return new Promise(resolve => {
-      content.navigator.mediaDevices
-        .getUserMedia({ video: true, fake: true })
-        .catch(resolve(false))
-        .then(resolve(true));
-    });
-  });
+  return SpecialPowers.spawn(browser, [], () =>
+    content.navigator.mediaDevices
+      .getUserMedia({ video: true, fake: true })
+      .then(() => true, () => false)
+  );
 }
 
 add_task(async function test_fullscreen_closes_permissionui_prompt() {
@@ -145,7 +142,7 @@ add_task(async function test_permission_prompt_closes_fullscreen() {
   let fullScreenExit = waitForFullScreenState(browser, false);
 
   info("Requesting notification permission");
-  requestNotificationPermission(browser);
+  requestNotificationPermission(browser).catch(() => {});
   await popupShown;
 
   info("Waiting for full-screen exit");
diff --git a/browser/base/content/test/general/browser_bug763468_perwindowpb.js b/browser/base/content/test/general/browser_bug763468_perwindowpb.js
index dff1f31da01e..78ab921ba91b 100644
--- a/browser/base/content/test/general/browser_bug763468_perwindowpb.js
+++ b/browser/base/content/test/general/browser_bug763468_perwindowpb.js
@@ -52,14 +52,10 @@ async function openNewTab(aWindow, aExpectedURL) {
     false,
     aExpectedURL
   );
-  let alreadyLoaded = await SpecialPowers.spawn(
-    browser,
-    [aExpectedURL],
-    url => {
-      let doc = content.document;
-      return doc && doc.readyState === "complete" && doc.location.href == url;
-    }
-  );
+  let alreadyLoaded = await ContentTask.spawn(browser, aExpectedURL, url => {
+    let doc = content.document;
+    return doc && doc.readyState === "complete" && doc.location.href == url;
+  });
   if (!alreadyLoaded) {
     await loadPromise;
   }
diff --git a/browser/base/content/test/general/browser_bug767836_perwindowpb.js b/browser/base/content/test/general/browser_bug767836_perwindowpb.js
index 2d13059d4cc9..46835e9f8c39 100644
--- a/browser/base/content/test/general/browser_bug767836_perwindowpb.js
+++ b/browser/base/content/test/general/browser_bug767836_perwindowpb.js
@@ -71,14 +71,10 @@ async function openNewTab(aWindow, aExpectedURL) {
     false,
     aExpectedURL
   );
-  let alreadyLoaded = await SpecialPowers.spawn(
-    browser,
-    [aExpectedURL],
-    url => {
-      let doc = content.document;
-      return doc && doc.readyState === "complete" && doc.location.href == url;
-    }
-  );
+  let alreadyLoaded = await ContentTask.spawn(browser, aExpectedURL, url => {
+    let doc = content.document;
+    return doc && doc.readyState === "complete" && doc.location.href == url;
+  });
   if (!alreadyLoaded) {
     await loadPromise;
   }
diff --git a/browser/base/content/test/general/browser_tabs_isActive.js b/browser/base/content/test/general/browser_tabs_isActive.js
index cfe8023c1fe9..d75bf8046cc3 100644
--- a/browser/base/content/test/general/browser_tabs_isActive.js
+++ b/browser/base/content/test/general/browser_tabs_isActive.js
@@ -24,9 +24,7 @@ function getParentTabState(aTab) {
 }
 
 function getChildTabState(aTab) {
-  return SpecialPowers.spawn(aTab.linkedBrowser, [], async function() {
-    return docShell.isActive;
-  });
+  return ContentTask.spawn(aTab.linkedBrowser, null, () => docShell.isActive);
 }
 
 function checkState(parentSide, childSide, value, message) {
diff --git a/browser/base/content/test/popups/browser_popup_frames.js b/browser/base/content/test/popups/browser_popup_frames.js
index 986a196d83e0..40fec3ebd7ff 100644
--- a/browser/base/content/test/popups/browser_popup_frames.js
+++ b/browser/base/content/test/popups/browser_popup_frames.js
@@ -41,7 +41,7 @@ add_task(async function test_opening_blocked_popups() {
 
   ok(notification, "Should have notification.");
 
-  await SpecialPowers.spawn(tab.linkedBrowser, [baseURL], async function(uri) {
+  await ContentTask.spawn(tab.linkedBrowser, baseURL, async function(uri) {
     let iframe = content.document.createElement("iframe");
     let pageHideHappened = ContentTaskUtils.waitForEvent(
       this,
@@ -59,7 +59,7 @@ add_task(async function test_opening_blocked_popups() {
   ok(notification, "Should still have notification");
 
   // Now navigate the subframe.
-  await SpecialPowers.spawn(tab.linkedBrowser, [], async function() {
+  await ContentTask.spawn(tab.linkedBrowser, null, async function() {
     let pageHideHappened = ContentTaskUtils.waitForEvent(
       this,
       "pagehide",
diff --git a/browser/base/content/test/webrtc/head.js b/browser/base/content/test/webrtc/head.js
index 58c96709590b..e177fee622c3 100644
--- a/browser/base/content/test/webrtc/head.js
+++ b/browser/base/content/test/webrtc/head.js
@@ -276,9 +276,9 @@ function expectObserverCalledOnClose(
 }
 
 function promiseMessage(aMessage, aAction, aCount = 1) {
-  let promise = SpecialPowers.spawn(
+  let promise = ContentTask.spawn(
     gBrowser.selectedBrowser,
-    [[aMessage, aCount]],
+    [aMessage, aCount],
     async function([expectedMessage, expectedCount]) {
       return new Promise(resolve => {
         function listenForMessage({ data }) {
@@ -509,10 +509,8 @@ async function reloadAndAssertClosedStreams() {
   await disableObserverVerification();
 
   let loadedPromise = BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
-  await SpecialPowers.spawn(
-    gBrowser.selectedBrowser,
-    [],
-    "() => content.location.reload()"
+  await ContentTask.spawn(gBrowser.selectedBrowser, null, () =>
+    content.location.reload()
   );
 
   await loadedPromise;
diff --git a/browser/components/aboutlogins/tests/browser/browser_loginSortOrderRestored.js b/browser/components/aboutlogins/tests/browser/browser_loginSortOrderRestored.js
index 25f416871c12..e6495f2814c8 100644
--- a/browser/components/aboutlogins/tests/browser/browser_loginSortOrderRestored.js
+++ b/browser/components/aboutlogins/tests/browser/browser_loginSortOrderRestored.js
@@ -23,9 +23,9 @@ add_task(async function test_sort_order_persisted() {
       url: "about:logins",
     },
     async function(browser) {
-      await SpecialPowers.spawn(
+      await ContentTask.spawn(
         browser,
-        [[TEST_LOGIN1, TEST_LOGIN2]],
+        [TEST_LOGIN1, TEST_LOGIN2],
         async function([testLogin1, testLogin2]) {
           let loginList = Cu.waiveXrays(
             content.document.querySelector("login-list")
@@ -65,9 +65,7 @@ add_task(async function test_sort_order_persisted() {
       url: "about:logins",
     },
     async function(browser) {
-      await SpecialPowers.spawn(browser, [TEST_LOGIN1], async function(
-        testLogin1
-      ) {
+      await ContentTask.spawn(browser, TEST_LOGIN1, async function(testLogin1) {
         let loginList = Cu.waiveXrays(
           content.document.querySelector("login-list")
         );
diff --git a/browser/components/extensions/test/browser/head.js b/browser/components/extensions/test/browser/head.js
index 90602b1397ca..93ff93a37098 100644
--- a/browser/components/extensions/test/browser/head.js
+++ b/browser/components/extensions/test/browser/head.js
@@ -303,7 +303,7 @@ async function focusButtonAndPressKey(key, elem, modifiers) {
 }
 
 var awaitBrowserLoaded = browser =>
-  SpecialPowers.spawn(browser, [], () => {
+  ContentTask.spawn(browser, null, () => {
     if (
       content.document.readyState !== "complete" ||
       content.document.documentURI === "about:blank"
diff --git a/browser/components/newtab/test/browser/browser_aboutwelcome.js b/browser/components/newtab/test/browser/browser_aboutwelcome.js
index 4bd7a381ee0c..022ea08cfe9b 100644
--- a/browser/components/newtab/test/browser/browser_aboutwelcome.js
+++ b/browser/components/newtab/test/browser/browser_aboutwelcome.js
@@ -38,9 +38,9 @@ async function test_trailhead_branch(
   );
   let browser = tab.linkedBrowser;
 
-  await SpecialPowers.spawn(
+  await ContentTask.spawn(
     browser,
-    [{ expectedSelectors, branchName, unexpectedSelectors }],
+    { expectedSelectors, branchName, unexpectedSelectors },
     async ({
       expectedSelectors: expected,
       branchName: branch,
diff --git a/browser/components/newtab/test/browser/browser_as_load_location.js b/browser/components/newtab/test/browser/browser_as_load_location.js
index e31870e88475..f11b6cf503fc 100644
--- a/browser/components/newtab/test/browser/browser_as_load_location.js
+++ b/browser/components/newtab/test/browser/browser_as_load_location.js
@@ -15,9 +15,9 @@ async function checkNewtabLoads(selector, message) {
   await waitForPreloaded(browser);
 
   // check what the content task thinks has been loaded.
-  let found = await SpecialPowers.spawn(
+  let found = await ContentTask.spawn(
     browser,
-    [selector],
+    selector,
     arg => content.document.querySelector(arg) !== null
   );
   ok(found, message);
diff --git a/browser/components/newtab/test/browser/head.js b/browser/components/newtab/test/browser/head.js
index 6f1299f0b1e2..5f0d5490dc1a 100644
--- a/browser/components/newtab/test/browser/head.js
+++ b/browser/components/newtab/test/browser/head.js
@@ -47,9 +47,9 @@ async function clearHistoryAndBookmarks() {
  * not necessarily have had all its javascript/render logic executed.
  */
 async function waitForPreloaded(browser) {
-  let readyState = await SpecialPowers.spawn(
+  let readyState = await ContentTask.spawn(
     browser,
-    [],
+    null,
     () => content.document.readyState
   );
   if (readyState !== "complete") {
diff --git a/browser/components/resistfingerprinting/test/browser/browser_spoofing_keyboard_event.js b/browser/components/resistfingerprinting/test/browser/browser_spoofing_keyboard_event.js
index 0209381f4643..9c5eeec8c022 100644
--- a/browser/components/resistfingerprinting/test/browser/browser_spoofing_keyboard_event.js
+++ b/browser/components/resistfingerprinting/test/browser/browser_spoofing_keyboard_event.js
@@ -1986,9 +1986,9 @@ async function testKeyEvent(aTab, aTestCase) {
   let allKeyEventPromises = [];
 
   for (let testEvent of testEvents) {
-    let keyEventPromise = SpecialPowers.spawn(
+    let keyEventPromise = ContentTask.spawn(
       aTab.linkedBrowser,
-      [{ testEvent, result: aTestCase.result, kSameKeyCodeAndCharCodeValue }],
+      { testEvent, result: aTestCase.result, kSameKeyCodeAndCharCodeValue },
       async aInput => {
         function verifyKeyboardEvent(
           aEvent,
@@ -2217,9 +2217,9 @@ add_task(async function runTestForSuppressModifierKeys() {
 
   for (let eventType of ["keydown", "keyup"]) {
     for (let modifierKey of ["Alt", "Shift", "Control"]) {
-      let testPromise = SpecialPowers.spawn(
+      let testPromise = ContentTask.spawn(
         tab.linkedBrowser,
-        [eventType],
+        eventType,
         async aEventType => {
           let inputBox = content.document.getElementById("test");
 
diff --git a/browser/components/sessionstore/test/browser_911547.js b/browser/components/sessionstore/test/browser_911547.js
index 046ddbece82c..14b979660686 100644
--- a/browser/components/sessionstore/test/browser_911547.js
+++ b/browser/components/sessionstore/test/browser_911547.js
@@ -38,7 +38,7 @@ add_task(async function test() {
 
   await loadedPromise;
 
-  await SpecialPowers.spawn(browser, [], function({}) {
+  await SpecialPowers.spawn(browser, [], function() {
     // eslint-disable-line
     // the data: URI inherits the CSP and the inline script needs to be blocked
     is(
@@ -56,7 +56,7 @@ add_task(async function test() {
   await promiseTabRestored(tab);
   browser = tab.linkedBrowser;
 
-  await SpecialPowers.spawn(browser, [], function({}) {
+  await SpecialPowers.spawn(browser, [], function() {
     // eslint-disable-line
     // the data: URI should be restored including the inherited CSP and the
     // inline script should be blocked.
diff --git a/browser/components/sessionstore/test/browser_frametree.js b/browser/components/sessionstore/test/browser_frametree.js
index 1de925660420..cca26bd2da46 100644
--- a/browser/components/sessionstore/test/browser_frametree.js
+++ b/browser/components/sessionstore/test/browser_frametree.js
@@ -27,8 +27,8 @@ add_task(async function test_frametree() {
   is(await countNonDynamicFrames(browser), 2, "two non-dynamic child frames");
 
   // Go back in history.
-  let pageShowPromise = SpecialPowers.spawn(browser, [], async () => {
-    return ContentTaskUtils.waitForEvent(this, "pageshow", true);
+  let pageShowPromise = ContentTask.spawn(browser, null, async () => {
+    await ContentTaskUtils.waitForEvent(this, "pageshow", true);
   });
   browser.goBack();
   await pageShowPromise;
diff --git a/browser/components/sessionstore/test/browser_send_async_message_oom.js b/browser/components/sessionstore/test/browser_send_async_message_oom.js
index 092dd0b19fc8..4c14ac90b6dc 100644
--- a/browser/components/sessionstore/test/browser_send_async_message_oom.js
+++ b/browser/components/sessionstore/test/browser_send_async_message_oom.js
@@ -41,7 +41,7 @@ add_task(async function() {
   // Open a browser, configure it to cause OOM.
   let newTab = BrowserTestUtils.addTab(gBrowser, "about:robots");
   let browser = newTab.linkedBrowser;
-  await SpecialPowers.spawn(browser, [], frameScript);
+  await ContentTask.spawn(browser, null, frameScript);
 
   let promiseReported = new Promise(resolve => {
     browser.messageManager.addMessageListener("SessionStore:error", resolve);
diff --git a/browser/components/sessionstore/test/browser_sessionHistory.js b/browser/components/sessionstore/test/browser_sessionHistory.js
index 4d25ee942e64..ff60776e382c 100644
--- a/browser/components/sessionstore/test/browser_sessionHistory.js
+++ b/browser/components/sessionstore/test/browser_sessionHistory.js
@@ -82,8 +82,8 @@ add_task(async function test_pageshow() {
   await promiseBrowserLoaded(browser);
 
   // Wait until shistory changes.
-  let pageShowPromise = SpecialPowers.spawn(browser, [], async () => {
-    return ContentTaskUtils.waitForEvent(this, "pageshow", true);
+  let pageShowPromise = ContentTask.spawn(browser, null, async () => {
+    await ContentTaskUtils.waitForEvent(this, "pageshow", true);
   });
 
   // Go back to the previous url which is loaded from the bfcache.
diff --git a/browser/components/sessionstore/test/browser_windowStateContainer.js b/browser/components/sessionstore/test/browser_windowStateContainer.js
index 1c339a6e5911..7249fb6b63c7 100644
--- a/browser/components/sessionstore/test/browser_windowStateContainer.js
+++ b/browser/components/sessionstore/test/browser_windowStateContainer.js
@@ -52,7 +52,7 @@ add_task(async function() {
 
   for (let i = 0; i < 4; i++) {
     let browser = win2.gBrowser.tabs[i].linkedBrowser;
-    await SpecialPowers.spawn(browser, [{ expectedId: i + 1 }], async function(
+    await ContentTask.spawn(browser, { expectedId: i + 1 }, async function(
       args
     ) {
       Assert.equal(
@@ -129,9 +129,7 @@ add_task(async function() {
 
   for (let i = 0; i < 2; i++) {
     let browser = win2.gBrowser.tabs[i].linkedBrowser;
-    await SpecialPowers.spawn(browser, [{ expectedId: i }], async function(
-      args
-    ) {
+    await ContentTask.spawn(browser, { expectedId: i }, async function(args) {
       Assert.equal(
         docShell.getOriginAttributes().userContextId,
         args.expectedId,
diff --git a/browser/components/urlbar/tests/browser/browser_locationBarCommand.js b/browser/components/urlbar/tests/browser/browser_locationBarCommand.js
index 8d8c338ce851..6fe9d80600e0 100644
--- a/browser/components/urlbar/tests/browser/browser_locationBarCommand.js
+++ b/browser/components/urlbar/tests/browser/browser_locationBarCommand.js
@@ -278,7 +278,7 @@ function promiseCheckChildNoFocusedElement(browser) {
     return null;
   }
 
-  return SpecialPowers.spawn(browser, [], async function() {
+  return ContentTask.spawn(browser, null, async function() {
     Assert.equal(
       Services.focus.focusedElement,
       null,
diff --git a/browser/components/urlbar/tests/browser/browser_switchToTabHavingURI_aOpenParams.js b/browser/components/urlbar/tests/browser/browser_switchToTabHavingURI_aOpenParams.js
index 7000d3136256..aa6360163973 100644
--- a/browser/components/urlbar/tests/browser/browser_switchToTabHavingURI_aOpenParams.js
+++ b/browser/components/urlbar/tests/browser/browser_switchToTabHavingURI_aOpenParams.js
@@ -13,7 +13,7 @@ add_task(async function test_ignoreFragment() {
   switchTab("about:home#1", true);
   switchTab("about:mozilla", true);
 
-  let hashChangePromise = SpecialPowers.spawn(
+  let hashChangePromise = ContentTask.spawn(
     tabRefAboutHome.linkedBrowser,
     [],
     async function() {
diff --git a/browser/modules/test/browser/browser_preloading_tab_moving.js b/browser/modules/test/browser/browser_preloading_tab_moving.js
index 3800d7a15582..08a2e0969f5b 100644
--- a/browser/modules/test/browser/browser_preloading_tab_moving.js
+++ b/browser/modules/test/browser/browser_preloading_tab_moving.js
@@ -72,7 +72,7 @@ add_task(async function moving_works() {
     return ContentTaskUtils.waitForCondition(() => {
       return content.document.readyState == "complete";
     });
-  });
+  }).catch(Cu.reportError);
 
   ok(true, "Managed to open a tab in the original window still.");
 
diff --git a/devtools/client/inspector/test/shared-head.js b/devtools/client/inspector/test/shared-head.js
index 98fcfb3017a2..3656fde7e62f 100644
--- a/devtools/client/inspector/test/shared-head.js
+++ b/devtools/client/inspector/test/shared-head.js
@@ -726,9 +726,9 @@ function openContextMenuAndGetAllItems(inspector, options) {
  */
 async function waitUntilVisitedState(tab, selectors) {
   await asyncWaitUntil(async () => {
-    const hasVisitedState = await SpecialPowers.spawn(
+    const hasVisitedState = await ContentTask.spawn(
       tab.linkedBrowser,
-      [selectors],
+      selectors,
       args => {
         const NS_EVENT_STATE_VISITED = 1 << 24;
 
diff --git a/devtools/client/responsive/test/browser/head.js b/devtools/client/responsive/test/browser/head.js
index 3f77cbe9a33f..d117b3fdcb43 100644
--- a/devtools/client/responsive/test/browser/head.js
+++ b/devtools/client/responsive/test/browser/head.js
@@ -194,7 +194,7 @@ function addRDMTask(rdmUrl, rdmTask, includeBrowserEmbeddedUI) {
 }
 
 function spawnViewportTask(ui, args, task) {
-  return SpecialPowers.spawn(ui.getViewportBrowser(), [args], task);
+  return ContentTask.spawn(ui.getViewportBrowser(), args, task);
 }
 
 function waitForFrameLoad(ui, targetURL) {
@@ -442,7 +442,7 @@ const selectNetworkThrottling = (ui, value) =>
   ]);
 
 function getSessionHistory(browser) {
-  return SpecialPowers.spawn(browser, [], async function() {
+  return ContentTask.spawn(browser, null, function() {
     /* eslint-disable no-undef */
     const { SessionHistory } = ChromeUtils.import(
       "resource://gre/modules/sessionstore/SessionHistory.jsm"
diff --git a/devtools/client/shared/test/browser_dbg_multiple-windows.js b/devtools/client/shared/test/browser_dbg_multiple-windows.js
index 6df6a58ba8a9..b04c42c4cfc3 100644
--- a/devtools/client/shared/test/browser_dbg_multiple-windows.js
+++ b/devtools/client/shared/test/browser_dbg_multiple-windows.js
@@ -75,7 +75,7 @@ async function testNewWindow(client, win) {
 
 async function testFocusFirst(client) {
   const tab = window.gBrowser.selectedTab;
-  await SpecialPowers.spawn(tab.linkedBrowser, [], async function() {
+  await ContentTask.spawn(tab.linkedBrowser, null, async function() {
     const onFocus = new Promise(resolve => {
       content.addEventListener("focus", resolve, { once: true });
     });
diff --git a/devtools/client/webconsole/test/browser/head.js b/devtools/client/webconsole/test/browser/head.js
index 82749965b7d3..5b6adec9fe12 100644
--- a/devtools/client/webconsole/test/browser/head.js
+++ b/devtools/client/webconsole/test/browser/head.js
@@ -1351,9 +1351,9 @@ async function selectFrame(dbg, frame) {
 async function pauseDebugger(dbg) {
   info("Waiting for debugger to pause");
   const onPaused = waitForPaused(dbg);
-  SpecialPowers.spawn(gBrowser.selectedBrowser, [], async function() {
+  SpecialPowers.spawn(gBrowser.selectedBrowser, [], function() {
     content.wrappedJSObject.firstCall();
-  });
+  }).catch(() => {});
   await onPaused;
 }
 
diff --git a/docshell/test/browser/browser_bug349769.js b/docshell/test/browser/browser_bug349769.js
index 0fe1263921b4..94ff94da3c43 100644
--- a/docshell/test/browser/browser_bug349769.js
+++ b/docshell/test/browser/browser_bug349769.js
@@ -2,7 +2,7 @@ add_task(async function test() {
   const uris = [undefined, "about:blank"];
 
   function checkContentProcess(newBrowser, uri) {
-    return SpecialPowers.spawn(newBrowser, [uri], async function(uri) {
+    return ContentTask.spawn(newBrowser, [uri], async function(uri) {
       var prin = content.document.nodePrincipal;
       Assert.notEqual(
         prin,
diff --git a/docshell/test/browser/browser_bug441169.js b/docshell/test/browser/browser_bug441169.js
index 3a29372f2b04..09e83b040c44 100644
--- a/docshell/test/browser/browser_bug441169.js
+++ b/docshell/test/browser/browser_bug441169.js
@@ -37,7 +37,7 @@ function test() {
   gBrowser.selectedTab = newTab;
   newBrowser = gBrowser.getBrowserForTab(newTab);
 
-  SpecialPowers.spawn(newBrowser, [], task).then(() => {
+  ContentTask.spawn(newBrowser, null, task).then(() => {
     gBrowser.removeCurrentTab();
     finish();
   });
diff --git a/docshell/test/browser/browser_bug670318.js b/docshell/test/browser/browser_bug670318.js
index 1dc14b518416..d6a685656c6b 100644
--- a/docshell/test/browser/browser_bug670318.js
+++ b/docshell/test/browser/browser_bug670318.js
@@ -16,7 +16,7 @@ add_task(async function test() {
     { gBrowser, url: "about:blank" },
     async function(browser) {
       if (!SpecialPowers.getBoolPref("fission.sessionHistoryInParent")) {
-        await SpecialPowers.spawn(browser, [URL], async function(URL) {
+        await ContentTask.spawn(browser, URL, async function(URL) {
           let history = docShell.QueryInterface(Ci.nsIWebNavigation)
             .sessionHistory;
           let count = 0;
@@ -29,22 +29,16 @@ add_task(async function test() {
           let listener = {
             OnHistoryNewEntry(aNewURI) {
               if (aNewURI.spec == URL && 5 == ++count) {
-                docShell.chromeEventHandler.addEventListener(
+                addEventListener(
                   "load",
                   function onLoad() {
-                    docShell.chromeEventHandler.removeEventListener(
-                      "load",
-                      onLoad,
-                      true
-                    );
-
                     Assert.ok(
                       history.index < history.count,
                       "history.index is valid"
                     );
                     testDone.resolve();
                   },
-                  true
+                  { capture: true, once: true }
                 );
 
                 history.legacySHistory.removeSHistoryListener(listener);
@@ -98,9 +92,9 @@ add_task(async function test() {
         async OnHistoryNewEntry(aNewURI) {
           if (aNewURI.spec == URL && 5 == ++count) {
             history.removeSHistoryListener(listener);
-            await SpecialPowers.spawn(browser, [], async () => {
-              let promise = new Promise(resolve => {
-                docShell.chromeEventHandler.addEventListener(
+            await ContentTask.spawn(browser, null, () => {
+              return new Promise(resolve => {
+                addEventListener(
                   "load",
                   evt => {
                     let history = docShell.QueryInterface(Ci.nsIWebNavigation)
@@ -113,10 +107,9 @@ add_task(async function test() {
                   },
                   { capture: true, once: true }
                 );
-              });
 
-              content.location.reload();
-              await promise;
+                content.location.reload();
+              });
             });
             testDone.resolve();
           }
diff --git a/docshell/test/browser/browser_bug673467.js b/docshell/test/browser/browser_bug673467.js
index 12bcb906ca0f..182cc0ee80b9 100644
--- a/docshell/test/browser/browser_bug673467.js
+++ b/docshell/test/browser/browser_bug673467.js
@@ -23,9 +23,9 @@ function test() {
   let taskFinished;
 
   let tab = BrowserTestUtils.addTab(gBrowser, doc, {}, tab => {
-    taskFinished = SpecialPowers.spawn(tab.linkedBrowser, [], () => {
+    taskFinished = ContentTask.spawn(tab.linkedBrowser, null, () => {
       return new Promise(resolve => {
-        docShell.chromeEventHandler.addEventListener(
+        addEventListener(
           "load",
           function() {
             // The main page has loaded.  Now wait for the iframe to load.
diff --git a/dom/base/test/browser_bug1058164.js b/dom/base/test/browser_bug1058164.js
index d93fa7de207c..40f8a8c132d8 100644
--- a/dom/base/test/browser_bug1058164.js
+++ b/dom/base/test/browser_bug1058164.js
@@ -203,7 +203,7 @@ add_task(async function test_swap_frameloader_pagevisibility_events() {
 
   // Wait for that initial browser to show its pageshow event if it hasn't
   // happened so that we don't confuse it with the other expected events.
-  await SpecialPowers.spawn(emptyBrowser, [], async () => {
+  await ContentTask.spawn(emptyBrowser, null, async () => {
     if (content.document.visibilityState === "hidden") {
       info("waiting for hidden emptyBrowser to pageshow");
       await ContentTaskUtils.waitForEvent(content, "pageshow", {});
diff --git a/dom/credentialmanagement/tests/browser/browser_active_document.js b/dom/credentialmanagement/tests/browser/browser_active_document.js
index ad56c1e7a9f1..2799979f6eaf 100644
--- a/dom/credentialmanagement/tests/browser/browser_active_document.js
+++ b/dom/credentialmanagement/tests/browser/browser_active_document.js
@@ -16,7 +16,7 @@ function expectNotAllowedError(aResult) {
 }
 
 function promiseMakeCredential(tab) {
-  return SpecialPowers.spawn(tab.linkedBrowser, [], async function() {
+  return ContentTask.spawn(tab.linkedBrowser, null, async function() {
     const cose_alg_ECDSA_w_SHA256 = -7;
 
     let publicKey = {
@@ -37,7 +37,7 @@ function promiseMakeCredential(tab) {
 }
 
 function promiseGetAssertion(tab) {
-  return SpecialPowers.spawn(tab.linkedBrowser, [], async function() {
+  return ContentTask.spawn(tab.linkedBrowser, null, async function() {
     let newCredential = {
       type: "public-key",
       id: content.crypto.getRandomValues(new Uint8Array(16)),
diff --git a/dom/html/test/browser_DOMDocElementInserted.js b/dom/html/test/browser_DOMDocElementInserted.js
index caef6aaa264d..241b0f93da6a 100644
--- a/dom/html/test/browser_DOMDocElementInserted.js
+++ b/dom/html/test/browser_DOMDocElementInserted.js
@@ -3,27 +3,16 @@ add_task(async function() {
   let tab = BrowserTestUtils.addTab(gBrowser);
   let uri = "data:text/html;charset=utf-8,<html/>";
 
-  let eventPromise = SpecialPowers.spawn(tab.linkedBrowser, [], function() {
-    const { PromiseUtils } = ChromeUtils.import(
-      "resource://gre/modules/PromiseUtils.jsm"
-    );
-    let deferred = PromiseUtils.defer();
-
-    let listener = event => {
-      docShell.chromeEventHandler.removeEventListener(
+  let eventPromise = ContentTask.spawn(tab.linkedBrowser, null, function() {
+    return new Promise(resolve => {
+      addEventListener(
         "DOMDocElementInserted",
-        listener,
-        true
+        event => resolve(event.target.documentURIObject.spec),
+        {
+          once: true,
+        }
       );
-      deferred.resolve(event.target.documentURIObject.spec);
-    };
-    docShell.chromeEventHandler.addEventListener(
-      "DOMDocElementInserted",
-      listener,
-      true
-    );
-
-    return deferred.promise;
+    });
   });
 
   BrowserTestUtils.loadURI(tab.linkedBrowser, uri);
diff --git a/dom/manifest/test/browser_fire_appinstalled_event.js b/dom/manifest/test/browser_fire_appinstalled_event.js
index 92fde29d6613..3dd721447fa3 100644
--- a/dom/manifest/test/browser_fire_appinstalled_event.js
+++ b/dom/manifest/test/browser_fire_appinstalled_event.js
@@ -23,7 +23,7 @@ function enableOnAppInstalledPref() {
 // This cause file_reg_install_event.html to be dynamically change.
 async function theTest(aBrowser) {
   aBrowser.allowEvents = true;
-  let waitForInstall = SpecialPowers.spawn(aBrowser, [], async function() {
+  let waitForInstall = ContentTask.spawn(aBrowser, null, async function() {
     await ContentTaskUtils.waitForEvent(content.window, "appinstalled");
   });
   const {
diff --git a/dom/security/test/csp/browser_test_web_manifest.js b/dom/security/test/csp/browser_test_web_manifest.js
index 727c17188c5a..3868eeabe910 100644
--- a/dom/security/test/csp/browser_test_web_manifest.js
+++ b/dom/security/test/csp/browser_test_web_manifest.js
@@ -223,7 +223,7 @@ function waitForNetObserver(aBrowser, aTest) {
     return Promise.resolve();
   }
 
-  return SpecialPowers.spawn(aBrowser, [], () => {
+  return ContentTask.spawn(aBrowser, [], () => {
     return new Promise(resolve => {
       function observe(subject, topic) {
         Services.obs.removeObserver(observe, "csp-on-violate-policy");
diff --git a/dom/tests/browser/browser_ConsoleStorageAPITests.js b/dom/tests/browser/browser_ConsoleStorageAPITests.js
index 4baf210988a2..e5b48eee4c06 100644
--- a/dom/tests/browser/browser_ConsoleStorageAPITests.js
+++ b/dom/tests/browser/browser_ConsoleStorageAPITests.js
@@ -24,7 +24,7 @@ add_task(async function() {
   gBrowser.selectedTab = tab;
   var browser = gBrowser.selectedBrowser;
 
-  let observerPromise = SpecialPowers.spawn(browser, [], async function(opt) {
+  let observerPromise = ContentTask.spawn(browser, null, async function(opt) {
     const TEST_URI =
       "http://example.com/browser/dom/tests/browser/test-console-api.html";
     let ConsoleAPIStorage = Cc["@mozilla.org/consoleAPI-storage;1"].getService(
@@ -93,9 +93,7 @@ add_task(async function() {
   browser = gBrowser.selectedBrowser;
 
   // Spin the event loop to make sure everything is cleared.
-  await SpecialPowers.spawn(browser, [], function() {
-    return Promise.resolve();
-  });
+  await SpecialPowers.spawn(browser, [], () => {});
 
   await SpecialPowers.spawn(browser, [windowId], function(windowId) {
     var ConsoleAPIStorage = Cc["@mozilla.org/consoleAPI-storage;1"].getService(
diff --git a/dom/tests/browser/browser_hasbeforeunload.js b/dom/tests/browser/browser_hasbeforeunload.js
index 2b2b8a2e97f1..204d76186428 100644
--- a/dom/tests/browser/browser_hasbeforeunload.js
+++ b/dom/tests/browser/browser_hasbeforeunload.js
@@ -313,7 +313,7 @@ function controlFrameAt(browser, frameDepth, command) {
         break;
       }
     }
-  });
+  }).catch(Cu.reportError);
 }
 
 /**
diff --git a/dom/tests/browser/helper_largeAllocation.js b/dom/tests/browser/helper_largeAllocation.js
index 90d1f00ac01c..0117e8500342 100644
--- a/dom/tests/browser/helper_largeAllocation.js
+++ b/dom/tests/browser/helper_largeAllocation.js
@@ -41,13 +41,13 @@ function expectNoProcess() {
 }
 
 function getPID(aBrowser) {
-  return SpecialPowers.spawn(aBrowser, [], () => {
+  return ContentTask.spawn(aBrowser, [], () => {
     return Services.appinfo.processID;
   });
 }
 
 function getInLAProc(aBrowser) {
-  return SpecialPowers.spawn(aBrowser, [], () => {
+  return ContentTask.spawn(aBrowser, [], () => {
     return Services.appinfo.remoteType == "webLargeAllocation";
   });
 }
@@ -80,7 +80,7 @@ async function largeAllocSuccessTests() {
     is(false, await getInLAProc(aBrowser));
 
     let epc = expectProcessCreated();
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
@@ -107,7 +107,7 @@ async function largeAllocSuccessTests() {
     // Fail the test if we create a process
     let stopExpectNoProcess = expectNoProcess();
 
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       // eslint-disable-next-line no-unsanitized/property
       content.document.body.innerHTML = `<iframe src='${TEST_URI}'></iframe>`;
 
@@ -138,7 +138,7 @@ async function largeAllocSuccessTests() {
     // Fail the test if we create a process
     let stopExpectNoProcess = expectNoProcess();
 
-    let loaded = SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    let loaded = ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.body.innerHTML = "<button>CLICK ME</button>";
 
       return new Promise(resolve => {
@@ -173,7 +173,7 @@ async function largeAllocSuccessTests() {
 
     let epc = expectProcessCreated();
 
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
@@ -186,7 +186,7 @@ async function largeAllocSuccessTests() {
 
     await BrowserTestUtils.browserLoaded(aBrowser);
 
-    await SpecialPowers.spawn(
+    await ContentTask.spawn(
       aBrowser,
       [],
       () => (content.document.location = "about:blank")
@@ -200,7 +200,7 @@ async function largeAllocSuccessTests() {
 
     epc = expectProcessCreated();
 
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
@@ -221,7 +221,7 @@ async function largeAllocSuccessTests() {
 
     let epc = expectProcessCreated();
 
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
@@ -235,7 +235,7 @@ async function largeAllocSuccessTests() {
     await BrowserTestUtils.browserLoaded(aBrowser);
 
     // Switch to about:blank, so we can navigate back
-    await SpecialPowers.spawn(aBrowser, [], () => {
+    await ContentTask.spawn(aBrowser, [], () => {
       content.document.location = "about:blank";
     });
 
@@ -251,7 +251,7 @@ async function largeAllocSuccessTests() {
 
     // Navigate back to the previous page. As the large alloation process was
     // left, it won't be in bfcache and will have to be loaded fresh.
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.window.history.back();
     });
 
@@ -276,7 +276,7 @@ async function largeAllocSuccessTests() {
       BrowserTestUtils.browserLoaded(aBrowser),
     ]);
 
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
@@ -289,7 +289,7 @@ async function largeAllocSuccessTests() {
 
     let epc = expectProcessCreated();
 
-    await SpecialPowers.spawn(aBrowser, [TEST_URI_2], TEST_URI_2 => {
+    await ContentTask.spawn(aBrowser, TEST_URI_2, TEST_URI_2 => {
       content.document.location = TEST_URI_2;
     });
 
@@ -327,7 +327,7 @@ async function largeAllocSuccessTests() {
       BrowserTestUtils.browserLoaded(aBrowser),
     ]);
 
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
@@ -340,7 +340,7 @@ async function largeAllocSuccessTests() {
 
     let stopExpectNoProcess = expectNoProcess();
 
-    await SpecialPowers.spawn(aBrowser, [], () => {
+    await ContentTask.spawn(aBrowser, [], () => {
       this.__newWindow = content.window.open("about:blank");
       content.document.location = "about:blank";
     });
@@ -354,7 +354,7 @@ async function largeAllocSuccessTests() {
 
     stopExpectNoProcess();
 
-    await SpecialPowers.spawn(aBrowser, [], () => {
+    await ContentTask.spawn(aBrowser, [], () => {
       ok(this.__newWindow, "The window should have been stored");
       this.__newWindow.close();
     });
@@ -372,7 +372,7 @@ async function largeAllocSuccessTests() {
       BrowserTestUtils.browserLoaded(aBrowser),
     ]);
 
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
@@ -390,7 +390,7 @@ async function largeAllocSuccessTests() {
       gBrowser,
       "about:blank"
     );
-    await SpecialPowers.spawn(aBrowser, [], () => {
+    await ContentTask.spawn(aBrowser, [], () => {
       this.__newWindow = content.window.open("about:blank");
     });
     await promiseTabOpened;
@@ -410,7 +410,7 @@ async function largeAllocSuccessTests() {
 
     stopExpectNoProcess();
 
-    await SpecialPowers.spawn(aBrowser, [], () => {
+    await ContentTask.spawn(aBrowser, [], () => {
       ok(this.__newWindow, "The window should have been stored");
       this.__newWindow.close();
     });
@@ -428,7 +428,7 @@ async function largeAllocSuccessTests() {
       expectProcessCreated(),
       BrowserTestUtils.browserLoaded(aBrowser),
     ]);
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
@@ -476,7 +476,7 @@ async function largeAllocSuccessTests() {
       BrowserTestUtils.browserLoaded(aBrowser),
     ]);
 
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
@@ -493,7 +493,7 @@ async function largeAllocSuccessTests() {
       is(false, await getInLAProc(aBrowser));
 
       let ready = Promise.all([BrowserTestUtils.browserLoaded(aBrowser)]);
-      await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+      await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
         content.document.location = TEST_URI;
       });
       await ready;
@@ -517,7 +517,7 @@ async function largeAllocSuccessTests() {
     // Fail the test if we create a process
     let stopExpectNoProcess = expectNoProcess();
 
-    await SpecialPowers.spawn(aBrowser, [], () => {
+    await ContentTask.spawn(aBrowser, [], () => {
       content.document.location = "view-source:http://example.com";
     });
 
@@ -546,7 +546,7 @@ async function largeAllocSuccessTests() {
       expectProcessCreated(),
       BrowserTestUtils.browserLoaded(aBrowser),
     ]);
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
@@ -588,7 +588,7 @@ async function largeAllocSuccessTests() {
       expectProcessCreated(),
       BrowserTestUtils.browserLoaded(aBrowser),
     ]);
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
@@ -600,7 +600,7 @@ async function largeAllocSuccessTests() {
     is(true, await getInLAProc(aBrowser));
 
     await Promise.all([
-      SpecialPowers.spawn(aBrowser, [], () => {
+      ContentTask.spawn(aBrowser, null, () => {
         content.document.querySelector("#submit").click();
       }),
       BrowserTestUtils.browserLoaded(aBrowser),
@@ -632,7 +632,7 @@ async function largeAllocFailTests() {
     // Fail the test if we create a process
     let stopExpectNoProcess = expectNoProcess();
 
-    await SpecialPowers.spawn(aBrowser, [TEST_URI], TEST_URI => {
+    await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
       content.document.location = TEST_URI;
     });
 
diff --git a/dom/webauthn/tests/browser/browser_fido_appid_extension.js b/dom/webauthn/tests/browser/browser_fido_appid_extension.js
index ceda86eb9358..782db266370a 100644
--- a/dom/webauthn/tests/browser/browser_fido_appid_extension.js
+++ b/dom/webauthn/tests/browser/browser_fido_appid_extension.js
@@ -52,7 +52,7 @@ function promiseU2FRegister(tab, app_id) {
 }
 
 function promiseWebAuthnRegister(tab, appid) {
-  return SpecialPowers.spawn(tab.linkedBrowser, [[appid]], ([appid]) => {
+  return ContentTask.spawn(tab.linkedBrowser, appid, appid => {
     const cose_alg_ECDSA_w_SHA256 = -7;
 
     let challenge = content.crypto.getRandomValues(new Uint8Array(16));
@@ -84,9 +84,9 @@ function promiseWebAuthnRegister(tab, appid) {
 }
 
 function promiseWebAuthnSign(tab, key_handle, extensions = {}) {
-  return SpecialPowers.spawn(
+  return ContentTask.spawn(
     tab.linkedBrowser,
-    [[key_handle, extensions]],
+    [key_handle, extensions],
     ([key_handle, extensions]) => {
       let challenge = content.crypto.getRandomValues(new Uint8Array(16));
 
diff --git a/dom/webauthn/tests/browser/browser_webauthn_prompts.js b/dom/webauthn/tests/browser/browser_webauthn_prompts.js
index f0fc74c0c36a..99e1744ea3d4 100644
--- a/dom/webauthn/tests/browser/browser_webauthn_prompts.js
+++ b/dom/webauthn/tests/browser/browser_webauthn_prompts.js
@@ -51,44 +51,40 @@ function verifyDirectCertificate(attestationObject) {
 
 function promiseWebAuthnRegister(tab, attestation = "indirect") {
   /* eslint-disable no-shadow */
-  return SpecialPowers.spawn(
-    tab.linkedBrowser,
-    [[attestation]],
-    ([attestation]) => {
-      const cose_alg_ECDSA_w_SHA256 = -7;
+  return ContentTask.spawn(tab.linkedBrowser, attestation, attestation => {
+    const cose_alg_ECDSA_w_SHA256 = -7;
 
-      let challenge = content.crypto.getRandomValues(new Uint8Array(16));
+    let challenge = content.crypto.getRandomValues(new Uint8Array(16));
 
-      let pubKeyCredParams = [
-        {
-          type: "public-key",
-          alg: cose_alg_ECDSA_w_SHA256,
-        },
-      ];
+    let pubKeyCredParams = [
+      {
+        type: "public-key",
+        alg: cose_alg_ECDSA_w_SHA256,
+      },
+    ];
 
-      let publicKey = {
-        rp: { id: content.document.domain, name: "none", icon: "none" },
-        user: {
-          id: new Uint8Array(),
-          name: "none",
-          icon: "none",
-          displayName: "none",
-        },
-        pubKeyCredParams,
-        attestation,
-        challenge,
-      };
+    let publicKey = {
+      rp: { id: content.document.domain, name: "none", icon: "none" },
+      user: {
+        id: new Uint8Array(),
+        name: "none",
+        icon: "none",
+        displayName: "none",
+      },
+      pubKeyCredParams,
+      attestation,
+      challenge,
+    };
 
-      return content.navigator.credentials
-        .create({ publicKey })
-        .then(cred => cred.response.attestationObject);
-    }
-  );
+    return content.navigator.credentials
+      .create({ publicKey })
+      .then(cred => cred.response.attestationObject);
+  });
   /* eslint-enable no-shadow */
 }
 
 function promiseWebAuthnSign(tab) {
-  return SpecialPowers.spawn(tab.linkedBrowser, [[]], () => {
+  return ContentTask.spawn(tab.linkedBrowser, null, () => {
     let challenge = content.crypto.getRandomValues(new Uint8Array(16));
     let key_handle = content.crypto.getRandomValues(new Uint8Array(16));
 
diff --git a/dom/xhr/tests/browser_xhr_onchange_leak.js b/dom/xhr/tests/browser_xhr_onchange_leak.js
index fc5f1655e15f..13845ec54570 100644
--- a/dom/xhr/tests/browser_xhr_onchange_leak.js
+++ b/dom/xhr/tests/browser_xhr_onchange_leak.js
@@ -18,7 +18,7 @@ add_task(async function test() {
     "http://mochi.test:8888/browser/dom/xhr/tests/browser_xhr_onchange_leak.html";
   let newTab = await BrowserTestUtils.openNewForegroundTab(gBrowser, url);
   let browser = gBrowser.selectedBrowser;
-  let done = await SpecialPowers.spawn(browser, [], async function() {
+  let done = await ContentTask.spawn(browser, null, async function() {
     let promise = ContentTaskUtils.waitForEvent(this, "DOMContentLoaded", true);
     content.location = "http://mochi.test:8888/";
     await promise;
diff --git a/js/xpconnect/tests/browser/browser_dead_object.js b/js/xpconnect/tests/browser/browser_dead_object.js
index ce2b0ea40395..b101c6484820 100644
--- a/js/xpconnect/tests/browser/browser_dead_object.js
+++ b/js/xpconnect/tests/browser/browser_dead_object.js
@@ -11,9 +11,9 @@ add_task(async function test() {
   let newTab = await BrowserTestUtils.openNewForegroundTab(gBrowser, url);
   let browser = gBrowser.selectedBrowser;
   let innerWindowId = browser.innerWindowID;
-  let contentDocDead = await SpecialPowers.spawn(
+  let contentDocDead = await ContentTask.spawn(
     browser,
-    [{ innerWindowId }],
+    { innerWindowId },
     async function(args) {
       let doc = content.document;
       let { TestUtils } = ChromeUtils.import(
diff --git a/js/xpconnect/tests/browser/browser_exception_leak.js b/js/xpconnect/tests/browser/browser_exception_leak.js
index 374a02bd491e..13df33783d25 100644
--- a/js/xpconnect/tests/browser/browser_exception_leak.js
+++ b/js/xpconnect/tests/browser/browser_exception_leak.js
@@ -12,9 +12,9 @@ add_task(async function test() {
   let browser = gBrowser.selectedBrowser;
   let innerWindowId = browser.innerWindowID;
 
-  let stackTraceEmpty = await SpecialPowers.spawn(
+  let stackTraceEmpty = await ContentTask.spawn(
     browser,
-    [{ innerWindowId }],
+    { innerWindowId },
     async function(args) {
       let { TestUtils } = ChromeUtils.import(
         "resource://testing-common/TestUtils.jsm"
diff --git a/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm b/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
index 5e5a53de0651..9c4781ae3cf7 100644
--- a/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
+++ b/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
@@ -30,6 +30,7 @@ const { OS } = ChromeUtils.import("resource://gre/modules/osfile.jsm");
 
 XPCOMUtils.defineLazyModuleGetters(this, {
   BrowserWindowTracker: "resource:///modules/BrowserWindowTracker.jsm",
+  ContentTask: "resource://testing-common/ContentTask.jsm",
   E10SUtils: "resource://gre/modules/E10SUtils.jsm",
 });
 
@@ -852,18 +853,14 @@ var BrowserTestUtils = {
 
     // We cannot use the regular BrowserTestUtils helper for waiting here, since that
     // would try to insert the preloaded browser, which would only break things.
-    await gBrowser.preloadedBrowser.ownerGlobal.SpecialPowers.spawn(
-      gBrowser.preloadedBrowser,
-      [],
-      async () => {
-        await ContentTaskUtils.waitForCondition(() => {
-          return (
-            this.content.document &&
-            this.content.document.readyState == "complete"
-          );
-        });
-      }
-    );
+    await ContentTask.spawn(gBrowser.preloadedBrowser, [], async () => {
+      await ContentTaskUtils.waitForCondition(() => {
+        return (
+          this.content.document &&
+          this.content.document.readyState == "complete"
+        );
+      });
+    });
   },
 
   /**
diff --git a/toolkit/components/passwordmgr/test/browser/browser_DOMFormHasPassword.js b/toolkit/components/passwordmgr/test/browser/browser_DOMFormHasPassword.js
index 550095cebdaa..1079b33eb793 100644
--- a/toolkit/components/passwordmgr/test/browser/browser_DOMFormHasPassword.js
+++ b/toolkit/components/passwordmgr/test/browser/browser_DOMFormHasPassword.js
@@ -92,7 +92,7 @@ function task(contentIds) {
 add_task(async function() {
   let tab = (gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser));
 
-  let promise = SpecialPowers.spawn(tab.linkedBrowser, [ids], task);
+  let promise = ContentTask.spawn(tab.linkedBrowser, ids, task);
   BrowserTestUtils.loadURI(
     tab.linkedBrowser,
     "data:text/html;charset=utf-8," +
diff --git a/toolkit/components/passwordmgr/test/browser/browser_DOMInputPasswordAdded.js b/toolkit/components/passwordmgr/test/browser/browser_DOMInputPasswordAdded.js
index 4ab79b6eed1a..bbab34801560 100644
--- a/toolkit/components/passwordmgr/test/browser/browser_DOMInputPasswordAdded.js
+++ b/toolkit/components/passwordmgr/test/browser/browser_DOMInputPasswordAdded.js
@@ -106,7 +106,7 @@ function task(contentConsts) {
 
 add_task(async function() {
   let tab = (gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser));
-  let promise = SpecialPowers.spawn(tab.linkedBrowser, [consts], task);
+  let promise = ContentTask.spawn(tab.linkedBrowser, consts, task);
   BrowserTestUtils.loadURI(
     tab.linkedBrowser,
     "data:text/html;charset=utf-8," +
diff --git a/toolkit/components/passwordmgr/test/browser/browser_context_menu.js b/toolkit/components/passwordmgr/test/browser/browser_context_menu.js
index b4d2691b58cf..2bb9645dfa9e 100644
--- a/toolkit/components/passwordmgr/test/browser/browser_context_menu.js
+++ b/toolkit/components/passwordmgr/test/browser/browser_context_menu.js
@@ -449,9 +449,7 @@ async function assertContextMenuFill(
     formId,
     unchangedSelector,
   };
-  let continuePromise = SpecialPowers.spawn(browser, [data], async function(
-    data
-  ) {
+  let continuePromise = ContentTask.spawn(browser, data, async function(data) {
     let {
       username,
       password,
diff --git a/toolkit/components/passwordmgr/test/browser/browser_context_menu_generated_password.js b/toolkit/components/passwordmgr/test/browser/browser_context_menu_generated_password.js
index 9de53f71bd6c..285405e445f1 100644
--- a/toolkit/components/passwordmgr/test/browser/browser_context_menu_generated_password.js
+++ b/toolkit/components/passwordmgr/test/browser/browser_context_menu_generated_password.js
@@ -301,9 +301,9 @@ add_task(async function fill_generated_password_with_matching_logins() {
       await openPasswordContextMenu(browser, passwordInputSelector);
 
       // Execute the command of the first login menuitem found at the context menu.
-      let passwordChangedPromise = SpecialPowers.spawn(
+      let passwordChangedPromise = ContentTask.spawn(
         browser,
-        [],
+        null,
         async function() {
           let passwordInput = content.document.getElementById(
             "form-basic-password"
diff --git a/toolkit/components/passwordmgr/test/browser/head.js b/toolkit/components/passwordmgr/test/browser/head.js
index 13ade3c61b84..a165f6e834f9 100644
--- a/toolkit/components/passwordmgr/test/browser/head.js
+++ b/toolkit/components/passwordmgr/test/browser/head.js
@@ -168,15 +168,13 @@ async function getFormSubmitResponseResult(
   { username = "#user", password = "#pass" } = {}
 ) {
   // default selectors are for the response page produced by formsubmit.sjs
-  let fieldValues = await SpecialPowers.spawn(
+  let fieldValues = await ContentTask.spawn(
     browser,
-    [
-      {
-        resultURL,
-        usernameSelector: username,
-        passwordSelector: password,
-      },
-    ],
+    {
+      resultURL,
+      usernameSelector: username,
+      passwordSelector: password,
+    },
     async function({ resultURL, usernameSelector, passwordSelector }) {
       await ContentTaskUtils.waitForCondition(() => {
         return (
diff --git a/toolkit/content/tests/browser/browser_resume_bkg_video_on_tab_hover.js b/toolkit/content/tests/browser/browser_resume_bkg_video_on_tab_hover.js
index 4725f4cb9f08..137f2eda55f6 100644
--- a/toolkit/content/tests/browser/browser_resume_bkg_video_on_tab_hover.js
+++ b/toolkit/content/tests/browser/browser_resume_bkg_video_on_tab_hover.js
@@ -92,9 +92,9 @@ function get_video_decoding_suspend_promise(browser, reload) {
 function get_video_decoding_resume_promise(browser) {
   let suspend = false;
   let reload = false;
-  return SpecialPowers.spawn(
+  return ContentTask.spawn(
     browser,
-    [{ suspend, reload }],
+    { suspend, reload },
     check_video_decoding_state
   );
 }
diff --git a/toolkit/content/tests/chrome/bug304188_window.xhtml b/toolkit/content/tests/chrome/bug304188_window.xhtml
index 26905d78411e..455fce1b74bf 100644
--- a/toolkit/content/tests/chrome/bug304188_window.xhtml
+++ b/toolkit/content/tests/chrome/bug304188_window.xhtml
@@ -18,7 +18,7 @@ find-menu appears in editor element which has had makeEditable() called but desi
 
   <script type="application/javascript"><![CDATA[
     const {BrowserTestUtils} = ChromeUtils.import("resource://testing-common/BrowserTestUtils.jsm");
-    ;
+    const {ContentTask} = ChromeUtils.import("resource://testing-common/ContentTask.jsm");
     ContentTask.setTestScope(window.arguments[0]);
 
     var gFindBar = null;
@@ -45,12 +45,9 @@ find-menu appears in editor element which has had makeEditable() called but desi
       info("Starting test with browser '" + browserId + "'");
       gBrowser = document.getElementById(browserId);
       gFindBar.browser = gBrowser;
-      let promise = SpecialPowers.spawn(gBrowser, [], async function() {
+      let promise = ContentTask.spawn(gBrowser, [], async function() {
         return new Promise(resolve => {
-          docShell.chromeEventHandler.addEventListener("DOMContentLoaded", function listener() {
-            docShell.chromeEventHandler.removeEventListener("DOMContentLoaded", listener);
-            resolve();
-          });
+          addEventListener("DOMContentLoaded", () => resolve(), { once: true });
         });
       });
       BrowserTestUtils.loadURI(gBrowser, "data:text/html;charset=utf-8,some%20random%20text");
@@ -59,7 +56,7 @@ find-menu appears in editor element which has had makeEditable() called but desi
     }
 
     async function onDocumentLoaded() {
-      await SpecialPowers.spawn(gBrowser, [], async function() {
+      await ContentTask.spawn(gBrowser, [], async function() {
         var edsession = content.docShell.editingSession;
         edsession.makeWindowEditable(content, "html", false, true, false);
         content.focus();
@@ -74,7 +71,7 @@ find-menu appears in editor element which has had makeEditable() called but desi
 
     async function enterStringIntoEditor(aString) {
       for (let i = 0; i < aString.length; i++) {
-        await SpecialPowers.spawn(gBrowser, [{ charCode: aString.charCodeAt(i) }], async function(args) {
+        await ContentTask.spawn(gBrowser, [{ charCode: aString.charCodeAt(i) }], async function(args) {
           let event = content.document.createEvent("KeyboardEvent");
           event.initKeyEvent("keypress", true, true, null, false, false,
                              false, false, 0, args.charCode);
diff --git a/toolkit/content/tests/chrome/bug360437_window.xhtml b/toolkit/content/tests/chrome/bug360437_window.xhtml
index f27234a93b2a..d2794a37c933 100644
--- a/toolkit/content/tests/chrome/bug360437_window.xhtml
+++ b/toolkit/content/tests/chrome/bug360437_window.xhtml
@@ -18,8 +18,8 @@
         title="360437 test">
 
   <script type="application/javascript"><![CDATA[
-    ;
     const {BrowserTestUtils} = ChromeUtils.import("resource://testing-common/BrowserTestUtils.jsm");
+    const {ContentTask} = ChromeUtils.import("resource://testing-common/ContentTask.jsm");
     ContentTask.setTestScope(window.arguments[0]);
 
     var gFindBar = null;
@@ -48,12 +48,9 @@
       gFindBar.browser = gBrowser;
 
       let loadedPromise = BrowserTestUtils.browserLoaded(gBrowser);
-      let contentLoadedPromise = SpecialPowers.spawn(gBrowser, [], async function() {
+      let contentLoadedPromise = ContentTask.spawn(gBrowser, null, async function() {
         return new Promise(resolve => {
-          docShell.chromeEventHandler.addEventListener("DOMContentLoaded", function listener() {
-            docShell.chromeEventHandler.removeEventListener("DOMContentLoaded", listener);
-            resolve();
-          });
+          addEventListener("DOMContentLoaded", () => resolve(), { once: true });
         });
       });
       BrowserTestUtils.loadURI(gBrowser, "data:text/html,<form><input id='input' type='text' value='text inside an input element'></form>");
diff --git a/toolkit/content/tests/chrome/findbar_entireword_window.xhtml b/toolkit/content/tests/chrome/findbar_entireword_window.xhtml
index 8b29f6536c2a..73191ec98aec 100644
--- a/toolkit/content/tests/chrome/findbar_entireword_window.xhtml
+++ b/toolkit/content/tests/chrome/findbar_entireword_window.xhtml
@@ -19,8 +19,7 @@
 
   <script type="application/javascript"><![CDATA[
     const {BrowserTestUtils} = ChromeUtils.import("resource://testing-common/BrowserTestUtils.jsm");
-    ;
-    ContentTask.setTestScope(window.arguments[0]);
+    const {ContentTask} = ChromeUtils.import("resource://testing-common/ContentTask.jsm");
 
     var gFindBar = null;
     var gBrowser;
@@ -123,14 +122,11 @@
       gBrowser = document.getElementById(browserId);
       gFindBar.browser = gBrowser;
 
-      let promise = SpecialPowers.spawn(gBrowser, [], async function() {
-        return new Promise(resolve => {
-          docShell.chromeEventHandler.addEventListener("DOMContentLoaded", function listener() {
-            docShell.chromeEventHandler.removeEventListener("DOMContentLoaded", listener);
-            resolve();
-          });
-        });
-      });
+      let promise = ContentTask.spawn(gBrowser, null, () =>
+        new Promise(resolve => {
+          addEventListener("DOMContentLoaded", () => resolve(), { once: true });
+        })
+      );
       BrowserTestUtils.loadURI(gBrowser, kBaseURL + "/sample_entireword_" + testName + ".html");
       await promise;
       await onDocumentLoaded(testName);
diff --git a/toolkit/content/tests/chrome/window_browser_drop.xhtml b/toolkit/content/tests/chrome/window_browser_drop.xhtml
index 593c7011aaba..5bb9da33e36b 100644
--- a/toolkit/content/tests/chrome/window_browser_drop.xhtml
+++ b/toolkit/content/tests/chrome/window_browser_drop.xhtml
@@ -10,12 +10,11 @@
 <script>
 <![CDATA[
 
-;
+const { ContentTask } = ChromeUtils.import("resource://testing-common/ContentTask.jsm");
 
 function dropOnRemoteBrowserAsync(browser, data, shouldExpectStateChange) {
   ContentTask.setTestScope(window); // Need this so is/isnot/ok are available inside the contenttask
-  return SpecialPowers.spawn(browser, [{data, shouldExpectStateChange}], async function({data, shouldExpectStateChange}) {
-    ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm");
+  return ContentTask.spawn(browser, {data, shouldExpectStateChange}, async function({data, shouldExpectStateChange}) {
     const {Services} = ChromeUtils.import("resource://gre/modules/Services.jsm");
 
     if (!content.document.documentElement) {
diff --git a/toolkit/mozapps/extensions/test/xpinstall/browser_badargs.js b/toolkit/mozapps/extensions/test/xpinstall/browser_badargs.js
index 48a956297ccd..568a1fe03fc7 100644
--- a/toolkit/mozapps/extensions/test/xpinstall/browser_badargs.js
+++ b/toolkit/mozapps/extensions/test/xpinstall/browser_badargs.js
@@ -7,9 +7,9 @@ function test() {
   var triggers = encodeURIComponent(JSON.stringify(TESTROOT + "amosigned.xpi"));
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, TESTROOT);
 
-  SpecialPowers.spawn(gBrowser.selectedBrowser, [], function() {
+  ContentTask.spawn(gBrowser.selectedBrowser, null, function() {
     return new Promise(resolve => {
-      docShell.chromeEventHandler.addEventListener(
+      addEventListener(
         "load",
         () => {
           content.addEventListener("InstallTriggered", () => {
diff --git a/toolkit/mozapps/extensions/test/xpinstall/browser_badargs2.js b/toolkit/mozapps/extensions/test/xpinstall/browser_badargs2.js
index e46b1f08ffb2..62a324b11f53 100644
--- a/toolkit/mozapps/extensions/test/xpinstall/browser_badargs2.js
+++ b/toolkit/mozapps/extensions/test/xpinstall/browser_badargs2.js
@@ -13,9 +13,9 @@ function test() {
   );
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, TESTROOT);
 
-  SpecialPowers.spawn(gBrowser.selectedBrowser, [], function() {
+  ContentTask.spawn(gBrowser.selectedBrowser, null, function() {
     return new Promise(resolve => {
-      docShell.chromeEventHandler.addEventListener(
+      addEventListener(
         "load",
         () => {
           content.addEventListener("InstallTriggered", () => {
diff --git a/toolkit/mozapps/extensions/test/xpinstall/browser_enabled.js b/toolkit/mozapps/extensions/test/xpinstall/browser_enabled.js
index 268ec0cc2820..08fd25645f8a 100644
--- a/toolkit/mozapps/extensions/test/xpinstall/browser_enabled.js
+++ b/toolkit/mozapps/extensions/test/xpinstall/browser_enabled.js
@@ -5,9 +5,9 @@ function test() {
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, TESTROOT);
 
-  SpecialPowers.spawn(
+  ContentTask.spawn(
     gBrowser.selectedBrowser,
-    [TESTROOT + "enabled.html"],
+    TESTROOT + "enabled.html",
     function(url) {
       return new Promise(resolve => {
         function page_loaded() {
@@ -16,19 +16,11 @@ function test() {
         }
 
         function load_listener() {
-          docShell.chromeEventHandler.removeEventListener(
-            "load",
-            load_listener,
-            true
-          );
+          removeEventListener("load", load_listener, true);
           content.addEventListener("PageLoaded", page_loaded);
         }
 
-        docShell.chromeEventHandler.addEventListener(
-          "load",
-          load_listener,
-          true
-        );
+        addEventListener("load", load_listener, true);
 
         content.location.href = url;
       });
diff --git a/toolkit/mozapps/extensions/test/xpinstall/browser_enabled2.js b/toolkit/mozapps/extensions/test/xpinstall/browser_enabled2.js
index 615dfd064b3f..ce37f68e8bd1 100644
--- a/toolkit/mozapps/extensions/test/xpinstall/browser_enabled2.js
+++ b/toolkit/mozapps/extensions/test/xpinstall/browser_enabled2.js
@@ -7,9 +7,9 @@ function test() {
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, TESTROOT);
 
-  SpecialPowers.spawn(
+  ContentTask.spawn(
     gBrowser.selectedBrowser,
-    [TESTROOT + "enabled.html"],
+    TESTROOT + "enabled.html",
     function(url) {
       return new Promise(resolve => {
         function page_loaded() {
@@ -18,19 +18,11 @@ function test() {
         }
 
         function load_listener() {
-          docShell.chromeEventHandler.removeEventListener(
-            "load",
-            load_listener,
-            true
-          );
+          removeEventListener("load", load_listener, true);
           content.addEventListener("PageLoaded", page_loaded);
         }
 
-        docShell.chromeEventHandler.addEventListener(
-          "load",
-          load_listener,
-          true
-        );
+        addEventListener("load", load_listener, true);
 
         content.location.href = url;
       });
diff --git a/toolkit/mozapps/extensions/test/xpinstall/browser_enabled3.js b/toolkit/mozapps/extensions/test/xpinstall/browser_enabled3.js
index 18e8b8202447..f207739c49b9 100644
--- a/toolkit/mozapps/extensions/test/xpinstall/browser_enabled3.js
+++ b/toolkit/mozapps/extensions/test/xpinstall/browser_enabled3.js
@@ -15,9 +15,9 @@ function test() {
   );
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, TESTROOT);
 
-  SpecialPowers.spawn(
+  ContentTask.spawn(
     gBrowser.selectedBrowser,
-    [TESTROOT + "installtrigger.html?" + triggers],
+    TESTROOT + "installtrigger.html?" + triggers,
     url => {
       return new Promise(resolve => {
         function page_loaded() {
@@ -26,19 +26,11 @@ function test() {
         }
 
         function load_listener() {
-          docShell.chromeEventHandler.removeEventListener(
-            "load",
-            load_listener,
-            true
-          );
+          removeEventListener("load", load_listener, true);
           content.addEventListener("InstallTriggered", page_loaded);
         }
 
-        docShell.chromeEventHandler.addEventListener(
-          "load",
-          load_listener,
-          true
-        );
+        addEventListener("load", load_listener, true);
 
         content.location.href = url;
       });
diff --git a/toolkit/mozapps/extensions/test/xpinstall/browser_localfile2.js b/toolkit/mozapps/extensions/test/xpinstall/browser_localfile2.js
index b97f65b5f343..b107e4e8d041 100644
--- a/toolkit/mozapps/extensions/test/xpinstall/browser_localfile2.js
+++ b/toolkit/mozapps/extensions/test/xpinstall/browser_localfile2.js
@@ -32,9 +32,13 @@ add_task(async function test() {
     await SpecialPowers.spawn(browser, [URI], async function(URI) {
       content.location.href = URI;
 
-      let loaded = ContentTaskUtils.waitForEvent(this, "load", true);
+      let loaded = ContentTaskUtils.waitForEvent(
+        docShell.chromeEventHandler,
+        "load",
+        true
+      );
       let installTriggered = ContentTaskUtils.waitForEvent(
-        this,
+        docShell.chromeEventHandler,
         "InstallTriggered",
         true,
         null,
diff --git a/toolkit/mozapps/extensions/test/xpinstall/browser_offline.js b/toolkit/mozapps/extensions/test/xpinstall/browser_offline.js
index 6b77f41cb3d7..3dfda36efed6 100644
--- a/toolkit/mozapps/extensions/test/xpinstall/browser_offline.js
+++ b/toolkit/mozapps/extensions/test/xpinstall/browser_offline.js
@@ -41,7 +41,7 @@ function finish_test(count) {
     info("Checking if the browser is still offline...");
 
     let tab = gBrowser.selectedTab;
-    SpecialPowers.spawn(tab.linkedBrowser, [], async function() {
+    ContentTask.spawn(tab.linkedBrowser, null, async function() {
       await ContentTaskUtils.waitForEvent(this, "DOMContentLoaded", true);
       return content.document.documentURI;
     }).then(url => {
diff --git a/tools/profiler/tests/browser/browser_test_feature_preferencereads.js b/tools/profiler/tests/browser/browser_test_feature_preferencereads.js
index 22b628095b8b..ff1b869a4738 100644
--- a/tools/profiler/tests/browser/browser_test_feature_preferencereads.js
+++ b/tools/profiler/tests/browser/browser_test_feature_preferencereads.js
@@ -72,16 +72,12 @@ add_task(async function test_profile_feature_preferencereads() {
 
     startProfiler({ features: ["threads", "leaf"] });
     // Now reload the tab with a clean run.
-    await SpecialPowers.spawn(contentBrowser, [], () => {
+    await ContentTask.spawn(contentBrowser, null, () => {
       return new Promise(resolve => {
-        docShell.chromeEventHandler.addEventListener(
-          "pageshow",
-          () => resolve(),
-          {
-            capturing: true,
-            once: true,
-          }
-        );
+        addEventListener("pageshow", () => resolve(), {
+          capturing: true,
+          once: true,
+        });
         content.location.reload();
       });
     });