diff --git a/.eslintignore b/.eslintignore
index 6749ce2ef7a5..4e45cb34759d 100644
--- a/.eslintignore
+++ b/.eslintignore
@@ -108,8 +108,6 @@ devtools/server/actors/object.js
 devtools/server/actors/script.js
 devtools/server/actors/styleeditor.js
 devtools/server/actors/stylesheets.js
-devtools/server/tests/browser/**
-!devtools/server/tests/browser/browser_webextension_inspected_window.js
 devtools/server/tests/mochitest/**
 devtools/server/tests/unit/**
 devtools/shared/heapsnapshot/**
diff --git a/devtools/server/tests/browser/animation.html b/devtools/server/tests/browser/animation.html
index 01847ac61446..3c58e55f2303 100644
--- a/devtools/server/tests/browser/animation.html
+++ b/devtools/server/tests/browser/animation.html
@@ -161,9 +161,9 @@
 <div class="multiple-animations-2"></div>
 <div class="all-transitions"></div>
 <script type="text/javascript">
+  "use strict";
   // Get the transitions started when the page loads
-  var players;
-  addEventListener("load", function() {
+  addEventListener("load", function () {
     document.querySelector(".transition").classList.add("get-round");
     document.querySelector(".delayed-transition").classList.add("get-round");
   });
diff --git a/devtools/server/tests/browser/browser_animation_getFrames.js b/devtools/server/tests/browser/browser_animation_getFrames.js
index 25ccfae3bb12..14d972520102 100644
--- a/devtools/server/tests/browser/browser_animation_getFrames.js
+++ b/devtools/server/tests/browser/browser_animation_getFrames.js
@@ -1,32 +1,30 @@
-/* vim: set ft=javascript ts=2 et sw=2 tw=80: */
-/* Any copyright is dedicated to the Public Domain.
-   http://creativecommons.org/publicdomain/zero/1.0/ */
-
-"use strict";
-
-// Check that the AnimationPlayerActor exposes a getFrames method that returns
-// the list of keyframes in the animation.
-
-const URL = MAIN_DOMAIN + "animation.html";
-
-add_task(function* () {
-  let {client, walker, animations} =
-    yield initAnimationsFrontForUrl(MAIN_DOMAIN + "animation.html");
-
-  info("Get the test node and its animation front");
-  let node = yield walker.querySelector(walker.rootNode, ".simple-animation");
-  let [player] = yield animations.getAnimationPlayersForNode(node);
-
-  ok(player.getFrames, "The front has the getFrames method");
-
-  let frames = yield player.getFrames();
-  is(frames.length, 2, "The correct number of keyframes was retrieved");
-  ok(frames[0].transform, "Frame 0 has the transform property");
-  ok(frames[1].transform, "Frame 1 has the transform property");
-  // Note that we don't really test the content of the frame object here on
-  // purpose. This object comes straight out of the web animations API
-  // unmodified.
-
-  yield client.close();
-  gBrowser.removeCurrentTab();
-});
+/* vim: set ft=javascript ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Check that the AnimationPlayerActor exposes a getFrames method that returns
+// the list of keyframes in the animation.
+
+add_task(function* () {
+  let {client, walker, animations} =
+    yield initAnimationsFrontForUrl(MAIN_DOMAIN + "animation.html");
+
+  info("Get the test node and its animation front");
+  let node = yield walker.querySelector(walker.rootNode, ".simple-animation");
+  let [player] = yield animations.getAnimationPlayersForNode(node);
+
+  ok(player.getFrames, "The front has the getFrames method");
+
+  let frames = yield player.getFrames();
+  is(frames.length, 2, "The correct number of keyframes was retrieved");
+  ok(frames[0].transform, "Frame 0 has the transform property");
+  ok(frames[1].transform, "Frame 1 has the transform property");
+  // Note that we don't really test the content of the frame object here on
+  // purpose. This object comes straight out of the web animations API
+  // unmodified.
+
+  yield client.close();
+  gBrowser.removeCurrentTab();
+});
diff --git a/devtools/server/tests/browser/browser_animation_getProperties.js b/devtools/server/tests/browser/browser_animation_getProperties.js
index e95db544c4a8..2bc12e7b0842 100644
--- a/devtools/server/tests/browser/browser_animation_getProperties.js
+++ b/devtools/server/tests/browser/browser_animation_getProperties.js
@@ -1,36 +1,36 @@
-/* vim: set ft=javascript ts=2 et sw=2 tw=80: */
-/* Any copyright is dedicated to the Public Domain.
-   http://creativecommons.org/publicdomain/zero/1.0/ */
-
-"use strict";
-
-// Check that the AnimationPlayerActor exposes a getProperties method that
-// returns the list of animated properties in the animation.
-
-const URL = MAIN_DOMAIN + "animation.html";
-
-add_task(function* () {
-  let {client, walker, animations} = yield initAnimationsFrontForUrl(URL);
-
-  info("Get the test node and its animation front");
-  let node = yield walker.querySelector(walker.rootNode, ".simple-animation");
-  let [player] = yield animations.getAnimationPlayersForNode(node);
-
-  ok(player.getProperties, "The front has the getProperties method");
-
-  let properties = yield player.getProperties();
-  is(properties.length, 1, "The correct number of properties was retrieved");
-
-  let propertyObject = properties[0];
-  is(propertyObject.name, "transform", "Property 0 is transform");
-
-  is(propertyObject.values.length, 2,
-    "The correct number of property values was retrieved");
-
-  // Note that we don't really test the content of the frame object here on
-  // purpose. This object comes straight out of the web animations API
-  // unmodified.
-
-  yield client.close();
-  gBrowser.removeCurrentTab();
-});
+/* vim: set ft=javascript ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Check that the AnimationPlayerActor exposes a getProperties method that
+// returns the list of animated properties in the animation.
+
+const URL = MAIN_DOMAIN + "animation.html";
+
+add_task(function* () {
+  let {client, walker, animations} = yield initAnimationsFrontForUrl(URL);
+
+  info("Get the test node and its animation front");
+  let node = yield walker.querySelector(walker.rootNode, ".simple-animation");
+  let [player] = yield animations.getAnimationPlayersForNode(node);
+
+  ok(player.getProperties, "The front has the getProperties method");
+
+  let properties = yield player.getProperties();
+  is(properties.length, 1, "The correct number of properties was retrieved");
+
+  let propertyObject = properties[0];
+  is(propertyObject.name, "transform", "Property 0 is transform");
+
+  is(propertyObject.values.length, 2,
+    "The correct number of property values was retrieved");
+
+  // Note that we don't really test the content of the frame object here on
+  // purpose. This object comes straight out of the web animations API
+  // unmodified.
+
+  yield client.close();
+  gBrowser.removeCurrentTab();
+});
diff --git a/devtools/server/tests/browser/browser_animation_refreshTransitions.js b/devtools/server/tests/browser/browser_animation_refreshTransitions.js
index 4cec0df69b61..4dcada91e1ba 100644
--- a/devtools/server/tests/browser/browser_animation_refreshTransitions.js
+++ b/devtools/server/tests/browser/browser_animation_refreshTransitions.js
@@ -21,8 +21,10 @@ add_task(function* () {
 
   info("Play a transition by adding the expand class, wait for mutations");
   let onMutations = expectMutationEvents(animations, 2);
-  let cpow = content.document.querySelector(".all-transitions");
-  cpow.classList.add("expand");
+  yield ContentTask.spawn(gBrowser.selectedBrowser, {}, () => {
+    let el = content.document.querySelector(".all-transitions");
+    el.classList.add("expand");
+  });
   let reportedMutations = yield onMutations;
 
   is(reportedMutations.length, 2, "2 mutation events were received");
@@ -35,7 +37,10 @@ add_task(function* () {
 
   info("Play the transition back by removing the class, wait for mutations");
   onMutations = expectMutationEvents(animations, 4);
-  cpow.classList.remove("expand");
+  yield ContentTask.spawn(gBrowser.selectedBrowser, {}, () => {
+    let el = content.document.querySelector(".all-transitions");
+    el.classList.remove("expand");
+  });
   reportedMutations = yield onMutations;
 
   is(reportedMutations.length, 4, "4 new mutation events were received");
diff --git a/devtools/server/tests/browser/browser_animation_setPlaybackRate.js b/devtools/server/tests/browser/browser_animation_setPlaybackRate.js
index b6d41b51e44d..4c10d134e094 100644
--- a/devtools/server/tests/browser/browser_animation_setPlaybackRate.js
+++ b/devtools/server/tests/browser/browser_animation_setPlaybackRate.js
@@ -41,9 +41,9 @@ add_task(function* () {
   yield animations.setPlaybackRates(players, .5);
 
   info("Query their states and check they are correct");
-  for (let player of players) {
-    let state = yield player.getCurrentState();
-    is(state.playbackRate, .5, "The playbackRate was updated");
+  for (let animPlayer of players) {
+    let animPlayerState = yield animPlayer.getCurrentState();
+    is(animPlayerState.playbackRate, .5, "The playbackRate was updated");
   }
 
   yield client.close();
diff --git a/devtools/server/tests/browser/browser_canvasframe_helper_01.js b/devtools/server/tests/browser/browser_canvasframe_helper_01.js
index 7fd9431977b2..76222833625e 100644
--- a/devtools/server/tests/browser/browser_canvasframe_helper_01.js
+++ b/devtools/server/tests/browser/browser_canvasframe_helper_01.js
@@ -19,6 +19,7 @@ const TEST_URL = "data:text/html;charset=utf-8,CanvasFrameAnonymousContentHelper
 
 add_task(function* () {
   let browser = yield addTab(TEST_URL);
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
   let doc = browser.contentDocument;
 
   let nodeBuilder = () => {
diff --git a/devtools/server/tests/browser/browser_canvasframe_helper_02.js b/devtools/server/tests/browser/browser_canvasframe_helper_02.js
index 90400c50a021..e60c238f37bf 100644
--- a/devtools/server/tests/browser/browser_canvasframe_helper_02.js
+++ b/devtools/server/tests/browser/browser_canvasframe_helper_02.js
@@ -19,6 +19,7 @@ const {
 
 add_task(function* () {
   let browser = yield addTab("about:preferences");
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
   let doc = browser.contentDocument;
 
   let nodeBuilder = () => {
diff --git a/devtools/server/tests/browser/browser_canvasframe_helper_03.js b/devtools/server/tests/browser/browser_canvasframe_helper_03.js
index 85e27c7deb59..e6016d65152a 100644
--- a/devtools/server/tests/browser/browser_canvasframe_helper_03.js
+++ b/devtools/server/tests/browser/browser_canvasframe_helper_03.js
@@ -20,6 +20,7 @@ const TEST_URL = "data:text/html;charset=utf-8,CanvasFrameAnonymousContentHelper
 
 add_task(function* () {
   let browser = yield addTab(TEST_URL);
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
   let doc = browser.contentDocument;
 
   let nodeBuilder = () => {
@@ -97,6 +98,6 @@ function synthesizeMouseDown(x, y, win) {
   // We need to make sure the inserted anonymous content can be targeted by the
   // event right after having been inserted, and so we need to force a sync
   // reflow.
-  let forceReflow = win.document.documentElement.offsetWidth;
+  win.document.documentElement.offsetWidth;
   EventUtils.synthesizeMouseAtPoint(x, y, {type: "mousedown"}, win);
 }
diff --git a/devtools/server/tests/browser/browser_canvasframe_helper_04.js b/devtools/server/tests/browser/browser_canvasframe_helper_04.js
index d038f84a0f43..3f70e5974eb7 100644
--- a/devtools/server/tests/browser/browser_canvasframe_helper_04.js
+++ b/devtools/server/tests/browser/browser_canvasframe_helper_04.js
@@ -10,7 +10,6 @@
 // This makes sure the 'domnode' protocol actor type is known when importing
 // highlighter.
 require("devtools/server/actors/inspector");
-const events = require("sdk/event/core");
 
 const {HighlighterEnvironment} = require("devtools/server/actors/highlighters");
 
@@ -18,11 +17,14 @@ const {
   CanvasFrameAnonymousContentHelper
 } = require("devtools/server/actors/highlighters/utils/markup");
 
-const TEST_URL_1 = "data:text/html;charset=utf-8,CanvasFrameAnonymousContentHelper test 1";
-const TEST_URL_2 = "data:text/html;charset=utf-8,CanvasFrameAnonymousContentHelper test 2";
+const TEST_URL_1 =
+  "data:text/html;charset=utf-8,CanvasFrameAnonymousContentHelper test 1";
+const TEST_URL_2 =
+  "data:text/html;charset=utf-8,CanvasFrameAnonymousContentHelper test 2";
 
 add_task(function* () {
-  let browser = yield addTab(TEST_URL_2);
+  let browser = yield addTab(TEST_URL_1);
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
   let doc = browser.contentDocument;
 
   let nodeBuilder = () => {
@@ -66,8 +68,9 @@ add_task(function* () {
 
   info("Navigating to a new page");
   let loaded = BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
-  content.location = TEST_URL_2;
+  BrowserTestUtils.loadURI(browser, TEST_URL_2);
   yield loaded;
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
   doc = gBrowser.selectedBrowser.contentWindow.document;
 
   info("Try to access the element again");
@@ -93,6 +96,6 @@ function synthesizeMouseDown(x, y, win) {
   // We need to make sure the inserted anonymous content can be targeted by the
   // event right after having been inserted, and so we need to force a sync
   // reflow.
-  let forceReflow = win.document.documentElement.offsetWidth;
+  win.document.documentElement.offsetWidth;
   EventUtils.synthesizeMouseAtPoint(x, y, {type: "mousedown"}, win);
 }
diff --git a/devtools/server/tests/browser/browser_canvasframe_helper_05.js b/devtools/server/tests/browser/browser_canvasframe_helper_05.js
index 94fb669145f0..13f5b4be3d90 100644
--- a/devtools/server/tests/browser/browser_canvasframe_helper_05.js
+++ b/devtools/server/tests/browser/browser_canvasframe_helper_05.js
@@ -21,6 +21,7 @@ const TEST_URL = "data:text/html;charset=utf-8,CanvasFrameAnonymousContentHelper
 
 add_task(function* () {
   let browser = yield addTab(TEST_URL);
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
   let doc = browser.contentDocument;
 
   let nodeBuilder = () => {
@@ -107,6 +108,6 @@ function synthesizeMouseDown(x, y, win) {
   // We need to make sure the inserted anonymous content can be targeted by the
   // event right after having been inserted, and so we need to force a sync
   // reflow.
-  let forceReflow = win.document.documentElement.offsetWidth;
+  win.document.documentElement.offsetWidth;
   EventUtils.synthesizeMouseAtPoint(x, y, {type: "mousedown"}, win);
 }
diff --git a/devtools/server/tests/browser/browser_canvasframe_helper_06.js b/devtools/server/tests/browser/browser_canvasframe_helper_06.js
index 2b137fe2638b..f4b4db232e1f 100644
--- a/devtools/server/tests/browser/browser_canvasframe_helper_06.js
+++ b/devtools/server/tests/browser/browser_canvasframe_helper_06.js
@@ -21,6 +21,7 @@ const TEST_URL = "data:text/html;charset=utf-8,CanvasFrameAnonymousContentHelper
 
 add_task(function* () {
   let browser = yield addTab(TEST_URL);
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
   let doc = browser.contentDocument;
 
   let nodeBuilder = () => {
@@ -95,6 +96,6 @@ function synthesizeMouseDown(x, y, win) {
   // We need to make sure the inserted anonymous content can be targeted by the
   // event right after having been inserted, and so we need to force a sync
   // reflow.
-  let forceReflow = win.document.documentElement.offsetWidth;
+  win.document.documentElement.offsetWidth;
   EventUtils.synthesizeMouseAtPoint(x, y, {type: "mousedown"}, win);
 }
diff --git a/devtools/server/tests/browser/browser_directorscript_actors.js b/devtools/server/tests/browser/browser_directorscript_actors.js
index d26c3b394c5c..cc201564cf54 100644
--- a/devtools/server/tests/browser/browser_directorscript_actors.js
+++ b/devtools/server/tests/browser/browser_directorscript_actors.js
@@ -8,8 +8,7 @@ const {DirectorManagerFront} = require("devtools/shared/fronts/director-manager"
 const {DirectorRegistry} = require("devtools/server/actors/director-registry");
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "director-script-target.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "director-script-target.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -50,8 +49,8 @@ function* testDirectorScriptMessagePort(directorManager) {
   let { port } = yield installAndEnableDirectorScript(directorManager, {
     scriptId: "testDirectorScript_MessagePort",
     scriptCode: "(" + (function () {
-      exports.attach = function ({port}) {
-        port.onmessage = function (evt) {
+      exports.attach = function ({port: messagePort}) {
+        messagePort.onmessage = function (evt) {
           // echo messages
           evt.target.postMessage(evt.data);
         };
@@ -69,22 +68,23 @@ function* testDirectorScriptMessagePort(directorManager) {
   // needs to explicit start the port
   port.start();
 
-  var msg = { k1: "v1", k2: [1, 2, 3] };
+  let msg = { k1: "v1", k2: [1, 2, 3] };
   port.postMessage(msg);
 
-  var reply = yield waitForMessagePortEvent;
+  let reply = yield waitForMessagePortEvent;
 
-  is(JSON.stringify(reply.data), JSON.stringify(msg), "echo reply received on the MessagePortClient");
+  is(JSON.stringify(reply.data), JSON.stringify(msg),
+    "echo reply received on the MessagePortClient");
 }
 
 function* testDirectorScriptWindowEval(directorManager) {
   let { port } = yield installAndEnableDirectorScript(directorManager, {
     scriptId: "testDirectorScript_WindowEval",
     scriptCode: "(" + (function () {
-      exports.attach = function ({window, port}) {
-        var onpageloaded = function () {
-          var globalVarValue = window.eval("globalAccessibleVar;");
-          port.postMessage(globalVarValue);
+      exports.attach = function ({window, port: evalPort}) {
+        let onpageloaded = function () {
+          let globalVarValue = window.eval("globalAccessibleVar;");
+          evalPort.postMessage(globalVarValue);
         };
 
         if (window.document && window.document.readyState === "complete") {
@@ -106,19 +106,20 @@ function* testDirectorScriptWindowEval(directorManager) {
   // needs to explicit start the port
   port.start();
 
-  var portEvent = yield waitForMessagePortEvent;
+  let portEvent = yield waitForMessagePortEvent;
 
   ok(portEvent.data !== "unsecure-eval", "window.eval should be wrapped and safe");
-  is(portEvent.data, "global-value", "globalAccessibleVar should be accessible through window.eval");
+  is(portEvent.data, "global-value",
+    "globalAccessibleVar should be accessible through window.eval");
 }
 
 function* testDirectorScriptUnloadOnDetach(directorManager) {
   let { port } = yield installAndEnableDirectorScript(directorManager, {
     scriptId: "testDirectorScript_unloadOnDetach",
     scriptCode: "(" + (function () {
-      exports.attach = function ({port, onUnload}) {
+      exports.attach = function ({port: unloadPort, onUnload}) {
         onUnload(function () {
-          port.postMessage("ONUNLOAD");
+          unloadPort.postMessage("ONUNLOAD");
         });
       };
     }).toString() + ")();",
@@ -133,7 +134,8 @@ function* testDirectorScriptUnloadOnDetach(directorManager) {
   let waitForDetach = once(directorManager, "director-script-detach");
   let waitForMessage = once(port, "message");
 
-  directorManager.disableByScriptIds(["testDirectorScript_unloadOnDetach"], {reload: false});
+  directorManager.disableByScriptIds(["testDirectorScript_unloadOnDetach"],
+                                    {reload: false});
 
   let { directorScriptId } = yield waitForDetach;
   is(directorScriptId, "testDirectorScript_unloadOnDetach",
diff --git a/devtools/server/tests/browser/browser_directorscript_actors_error_events.js b/devtools/server/tests/browser/browser_directorscript_actors_error_events.js
index 0afe1638875d..cdcf0157284f 100644
--- a/devtools/server/tests/browser/browser_directorscript_actors_error_events.js
+++ b/devtools/server/tests/browser/browser_directorscript_actors_error_events.js
@@ -8,8 +8,7 @@ const {DirectorManagerFront} = require("devtools/shared/fronts/director-manager"
 const {DirectorRegistry} = require("devtools/server/actors/director-registry");
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "director-script-target.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "director-script-target.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -43,12 +42,13 @@ function* testErrorOnRequire(directorManager) {
   assertIsDirectorScriptError(errorOnRequire);
 
   let { message } = errorOnRequire;
-  is(message, "Error: NOT IMPLEMENTED", "error.message contains the expected error message");
+  is(message, "Error: NOT IMPLEMENTED",
+    "error.message contains the expected error message");
 }
 
 function* testErrorOnEvaluate(directorManager) {
-  // director scripts should send an error events if the director script raise an exception on
-  // evaluation
+  // director scripts should send an error events if the director script
+  // raise an exception on evaluation
   let errorOnEvaluate = yield installAndEnableDirectorScript(directorManager, {
     scriptId: "testDirectorScript_errorOnEvaluate",
     scriptCode: "(" + (function () {
@@ -62,8 +62,8 @@ function* testErrorOnEvaluate(directorManager) {
 }
 
 function* testErrorOnAttach(directorManager) {
-  // director scripts should send an error events if the director script raise an exception on
-  // evaluation
+  // director scripts should send an error events if the director script
+  // raise an exception on evaluation
   let errorOnAttach = yield installAndEnableDirectorScript(directorManager, {
     scriptId: "testDirectorScript_errorOnAttach",
     scriptCode: "(" + (function () {
@@ -79,9 +79,9 @@ function* testErrorOnAttach(directorManager) {
 }
 
 function* testErrorOnDetach(directorManager) {
-  // director scripts should send an error events if the director script raise an exception on
-  // evaluation
-  let attach = yield installAndEnableDirectorScript(directorManager, {
+  // director scripts should send an error events if the director script
+  // raise an exception on evaluation
+  yield installAndEnableDirectorScript(directorManager, {
     scriptId: "testDirectorScript_errorOnDetach",
     scriptCode: "(" + (function () {
       module.exports = function ({onUnload}) {
@@ -97,7 +97,8 @@ function* testErrorOnDetach(directorManager) {
   let waitForDetach = once(directorManager, "director-script-detach");
   let waitForError = once(directorManager, "director-script-error");
 
-  directorManager.disableByScriptIds(["testDirectorScript_errorOnDetach"], {reload: false});
+  directorManager.disableByScriptIds(["testDirectorScript_errorOnDetach"],
+                                    {reload: false});
 
   let detach = yield waitForDetach;
   let error = yield waitForError;
diff --git a/devtools/server/tests/browser/browser_directorscript_actors_exports.js b/devtools/server/tests/browser/browser_directorscript_actors_exports.js
index f9ef56f51036..adb9695d6c75 100644
--- a/devtools/server/tests/browser/browser_directorscript_actors_exports.js
+++ b/devtools/server/tests/browser/browser_directorscript_actors_exports.js
@@ -10,8 +10,7 @@ const {DirectorRegistry} = require("devtools/server/actors/director-registry");
 DirectorRegistry.clear();
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "director-script-target.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "director-script-target.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
diff --git a/devtools/server/tests/browser/browser_markers-cycle-collection.js b/devtools/server/tests/browser/browser_markers-cycle-collection.js
index dc33375f2299..672d3a63bb06 100644
--- a/devtools/server/tests/browser/browser_markers-cycle-collection.js
+++ b/devtools/server/tests/browser/browser_markers-cycle-collection.js
@@ -5,6 +5,7 @@
  * Test that we get "nsCycleCollector::Collect" and
  * "nsCycleCollector::ForgetSkippable" markers when we force cycle collection.
  */
+"use strict";
 
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
 
@@ -12,8 +13,7 @@ add_task(function* () {
   // This test runs very slowly on linux32 debug EC2 instances.
   requestLongerTimeout(2);
 
-  let browser = yield addTab(MAIN_DOMAIN + "doc_force_cc.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_force_cc.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -22,11 +22,14 @@ add_task(function* () {
   yield front.connect();
   let rec = yield front.startRecording({ withMarkers: true });
 
-  let markers = yield waitForMarkerType(front, ["nsCycleCollector::Collect", "nsCycleCollector::ForgetSkippable"]);
+  let markers = yield waitForMarkerType(front,
+    ["nsCycleCollector::Collect", "nsCycleCollector::ForgetSkippable"]);
   yield front.stopRecording(rec);
 
-  ok(markers.some(m => m.name === "nsCycleCollector::Collect"), "got some nsCycleCollector::Collect markers");
-  ok(markers.some(m => m.name === "nsCycleCollector::ForgetSkippable"), "got some nsCycleCollector::Collect markers");
+  ok(markers.some(m => m.name === "nsCycleCollector::Collect"),
+    "got some nsCycleCollector::Collect markers");
+  ok(markers.some(m => m.name === "nsCycleCollector::ForgetSkippable"),
+    "got some nsCycleCollector::Collect markers");
 
   yield client.close();
   gBrowser.removeCurrentTab();
diff --git a/devtools/server/tests/browser/browser_markers-docloading-01.js b/devtools/server/tests/browser/browser_markers-docloading-01.js
index 3c82d56c4545..efb86c23aef6 100644
--- a/devtools/server/tests/browser/browser_markers-docloading-01.js
+++ b/devtools/server/tests/browser/browser_markers-docloading-01.js
@@ -4,12 +4,14 @@
 /**
  * Test that we get DOMContentLoaded and Load markers
  */
+"use strict";
 
 const { TimelineFront } = require("devtools/shared/fronts/timeline");
 const MARKER_NAMES = ["document::DOMContentLoaded", "document::Load"];
 
 add_task(function* () {
   let browser = yield addTab(MAIN_DOMAIN + "doc_innerHTML.html");
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
   let doc = browser.contentDocument;
 
   initDebuggerServer();
diff --git a/devtools/server/tests/browser/browser_markers-docloading-02.js b/devtools/server/tests/browser/browser_markers-docloading-02.js
index 0142ea0cdd22..14c60fc1e21d 100644
--- a/devtools/server/tests/browser/browser_markers-docloading-02.js
+++ b/devtools/server/tests/browser/browser_markers-docloading-02.js
@@ -4,12 +4,14 @@
 /**
  * Test that we get DOMContentLoaded and Load markers
  */
+"use strict";
 
 const { TimelineFront } = require("devtools/shared/fronts/timeline");
 const MARKER_NAMES = ["document::DOMContentLoaded", "document::Load"];
 
 add_task(function* () {
   let browser = yield addTab(MAIN_DOMAIN + "doc_innerHTML.html");
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
   let doc = browser.contentDocument;
 
   initDebuggerServer();
diff --git a/devtools/server/tests/browser/browser_markers-docloading-03.js b/devtools/server/tests/browser/browser_markers-docloading-03.js
index 1960da4daba7..2f31bcc99343 100644
--- a/devtools/server/tests/browser/browser_markers-docloading-03.js
+++ b/devtools/server/tests/browser/browser_markers-docloading-03.js
@@ -4,12 +4,14 @@
 /**
  * Test that we get DOMContentLoaded and Load markers
  */
+"use strict";
 
 const { TimelineFront } = require("devtools/shared/fronts/timeline");
 const MARKER_NAMES = ["document::DOMContentLoaded", "document::Load"];
 
 add_task(function* () {
   let browser = yield addTab(MAIN_DOMAIN + "doc_innerHTML.html");
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
   let doc = browser.contentDocument;
 
   initDebuggerServer();
diff --git a/devtools/server/tests/browser/browser_markers-gc.js b/devtools/server/tests/browser/browser_markers-gc.js
index 559b19161ed3..bf500664b39a 100644
--- a/devtools/server/tests/browser/browser_markers-gc.js
+++ b/devtools/server/tests/browser/browser_markers-gc.js
@@ -4,13 +4,13 @@
 /**
  * Test that we get "GarbageCollection" markers.
  */
+"use strict";
 
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
 const MARKER_NAME = "GarbageCollection";
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "doc_force_gc.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_force_gc.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -37,7 +37,8 @@ add_task(function* () {
       return current.start;
     }
     if (current.start < previousStart) {
-      ok(false, `markers must be in order. ${current.name} marker has later start time (${current.start}) thanprevious: ${previousStart}`);
+      ok(false, `markers must be in order. ${current.name} marker has later\
+        start time (${current.start}) thanprevious: ${previousStart}`);
       ordered = false;
     }
     return current.start;
diff --git a/devtools/server/tests/browser/browser_markers-minor-gc.js b/devtools/server/tests/browser/browser_markers-minor-gc.js
index 3327643485e4..b8802aa8a68b 100644
--- a/devtools/server/tests/browser/browser_markers-minor-gc.js
+++ b/devtools/server/tests/browser/browser_markers-minor-gc.js
@@ -5,6 +5,7 @@
  * Test that we get "MinorGC" markers when we continue to steadily allocate
  * objects.
  */
+"use strict";
 
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
 
@@ -12,7 +13,7 @@ add_task(function* () {
   // This test runs very slowly on linux32 debug EC2 instances.
   requestLongerTimeout(2);
 
-  let doc = yield addTab(MAIN_DOMAIN + "doc_allocations.html");
+  yield addTab(MAIN_DOMAIN + "doc_allocations.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
diff --git a/devtools/server/tests/browser/browser_markers-parse-html.js b/devtools/server/tests/browser/browser_markers-parse-html.js
index bd4f479c0ab8..5108ca8c4d76 100644
--- a/devtools/server/tests/browser/browser_markers-parse-html.js
+++ b/devtools/server/tests/browser/browser_markers-parse-html.js
@@ -4,13 +4,13 @@
 /**
  * Test that we get "Parse HTML" markers.
  */
+"use strict";
 
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
 const MARKER_NAME = "Parse HTML";
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "doc_innerHTML.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_innerHTML.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
diff --git a/devtools/server/tests/browser/browser_markers-styles.js b/devtools/server/tests/browser/browser_markers-styles.js
index a3dffe8b502a..656a27e722f5 100644
--- a/devtools/server/tests/browser/browser_markers-styles.js
+++ b/devtools/server/tests/browser/browser_markers-styles.js
@@ -4,13 +4,13 @@
 /**
  * Test that we get "Styles" markers with correct meta.
  */
+"use strict";
 
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
 const MARKER_NAME = "Styles";
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "doc_perf.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_perf.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -19,8 +19,8 @@ add_task(function* () {
   yield front.connect();
   let rec = yield front.startRecording({ withMarkers: true });
 
-  let markers = yield waitForMarkerType(front, MARKER_NAME, function (markers) {
-    return markers.some(({restyleHint}) => restyleHint != void 0);
+  let markers = yield waitForMarkerType(front, MARKER_NAME, function (marker) {
+    return marker.some(({restyleHint}) => restyleHint != void 0);
   });
 
   yield front.stopRecording(rec);
diff --git a/devtools/server/tests/browser/browser_markers-timestamp.js b/devtools/server/tests/browser/browser_markers-timestamp.js
index 428499502156..255eb05cb7fc 100644
--- a/devtools/server/tests/browser/browser_markers-timestamp.js
+++ b/devtools/server/tests/browser/browser_markers-timestamp.js
@@ -4,14 +4,15 @@
 /**
  * Test that we get a "TimeStamp" marker.
  */
+"use strict";
 
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
-const { pmmConsoleMethod, pmmLoadFrameScripts, pmmClearFrameScripts } = require("devtools/client/performance/test/helpers/profiler-mm-utils");
+const { pmmConsoleMethod, pmmLoadFrameScripts, pmmClearFrameScripts }
+  = require("devtools/client/performance/test/helpers/profiler-mm-utils");
 const MARKER_NAME = "TimeStamp";
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "doc_perf.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_perf.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -24,12 +25,14 @@ add_task(function* () {
   pmmConsoleMethod("timeStamp");
   pmmConsoleMethod("timeStamp", "myLabel");
 
-  let markers = yield waitForMarkerType(front, MARKER_NAME, markers => markers.length >= 2);
+  let markers = yield waitForMarkerType(front, MARKER_NAME, m => m.length >= 2);
 
   yield front.stopRecording(rec);
 
-  ok(markers.every(({stack}) => typeof stack === "number"), "All markers have stack references.");
-  ok(markers.every(({name}) => name === "TimeStamp"), "All markers found are TimeStamp markers");
+  ok(markers.every(({stack}) => typeof stack === "number"),
+    "All markers have stack references.");
+  ok(markers.every(({name}) => name === "TimeStamp"),
+    "All markers found are TimeStamp markers");
   ok(markers.length === 2, "found 2 TimeStamp markers");
   ok(markers.every(({start, end}) => typeof start === "number" && start === end),
     "All markers have equal start and end times");
diff --git a/devtools/server/tests/browser/browser_navigateEvents.js b/devtools/server/tests/browser/browser_navigateEvents.js
index 80b9d3fa661a..56aba69273ff 100644
--- a/devtools/server/tests/browser/browser_navigateEvents.js
+++ b/devtools/server/tests/browser/browser_navigateEvents.js
@@ -10,50 +10,56 @@ const URL2 = MAIN_DOMAIN + "navigate-second.html";
 var events = require("sdk/event/core");
 var client;
 
-SpecialPowers.pushPrefEnv({"set": [["dom.require_user_interaction_for_beforeunload", false]]});
+SpecialPowers.pushPrefEnv(
+  {"set": [["dom.require_user_interaction_for_beforeunload", false]]});
 
 // State machine to check events order
 var i = 0;
 function assertEvent(event, data) {
-  let x = 0;
   switch (i++) {
-    case x++:
+    case 0:
       is(event, "request", "Get first page load");
       is(data, URL1);
       break;
-    case x++:
+    case 1:
       is(event, "load-new-document", "Ask to load the second page");
       break;
-    case x++:
+    case 2:
       is(event, "unload-dialog", "We get the dialog on first page unload");
       break;
-    case x++:
+    case 3:
       is(event, "will-navigate", "The very first event is will-navigate on server side");
       is(data.newURI, URL2, "newURI property is correct");
       break;
-    case x++:
-      is(event, "request", "RDP is async with messageManager, the request happens after will-navigate");
+    case 4:
+      is(event, "request",
+        "RDP is async with messageManager, the request happens after will-navigate");
       is(data, URL2);
       break;
-    case x++:
+    case 5:
       is(event, "tabNavigated", "After the request, the client receive tabNavigated");
       is(data.state, "start", "state is start");
       is(data.url, URL2, "url property is correct");
       is(data.nativeConsoleAPI, true, "nativeConsoleAPI is correct");
       break;
-    case x++:
+    case 6:
       is(event, "DOMContentLoaded");
+      // eslint-disable-next-line mozilla/no-cpows-in-tests
       is(content.document.readyState, "interactive");
       break;
-    case x++:
+    case 7:
       is(event, "load");
+      // eslint-disable-next-line mozilla/no-cpows-in-tests
       is(content.document.readyState, "complete");
       break;
-    case x++:
-      is(event, "navigate", "Then once the second doc is loaded, we get the navigate event");
-      is(content.document.readyState, "complete", "navigate is emitted only once the document is fully loaded");
+    case 8:
+      is(event, "navigate",
+        "Then once the second doc is loaded, we get the navigate event");
+      // eslint-disable-next-line mozilla/no-cpows-in-tests
+      is(content.document.readyState, "complete",
+        "navigate is emitted only once the document is fully loaded");
       break;
-    case x++:
+    case 9:
       is(event, "tabNavigated", "Finally, the receive the client event");
       is(data.state, "stop", "state is stop");
       is(data.url, URL2, "url property is correct");
@@ -102,22 +108,21 @@ function getServerTabActor(callback) {
   client = new DebuggerClient(transport);
   connectDebuggerClient(client).then(form => {
     let actorID = form.actor;
-    client.attachTab(actorID, function (aResponse, aTabClient) {
+    client.attachTab(actorID, function (response, tabClient) {
       // !Hack! Retrieve a server side object, the BrowserTabActor instance
       let tabActor = DebuggerServer._searchAllConnectionsForActor(actorID);
       callback(tabActor);
     });
   });
 
-  client.addListener("tabNavigated", function (aEvent, aPacket) {
-    assertEvent("tabNavigated", aPacket);
+  client.addListener("tabNavigated", function (event, packet) {
+    assertEvent("tabNavigated", packet);
   });
 }
 
 function test() {
   // Open a test tab
   addTab(URL1).then(function (browser) {
-    let doc = browser.contentDocument;
     getServerTabActor(function (tabActor) {
       // In order to listen to internal will-navigate/navigate events
       events.on(tabActor, "will-navigate", function (data) {
@@ -140,9 +145,8 @@ function test() {
 
       // Load another document in this doc to dispatch these events
       assertEvent("load-new-document");
-      content.location = URL2;
+      BrowserTestUtils.loadURI(gBrowser.selectedBrowser, URL2);
     });
-
   });
 }
 
diff --git a/devtools/server/tests/browser/browser_perf-allocation-data.js b/devtools/server/tests/browser/browser_perf-allocation-data.js
index 3d4a94dee23a..9ea51e705315 100644
--- a/devtools/server/tests/browser/browser_perf-allocation-data.js
+++ b/devtools/server/tests/browser/browser_perf-allocation-data.js
@@ -5,11 +5,12 @@
  * Test that we have allocation data coming from the front.
  */
 
+"use strict";
+
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "doc_allocations.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_allocations.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -17,7 +18,8 @@ add_task(function* () {
   let front = PerformanceFront(client, form);
   yield front.connect();
 
-  let rec = yield front.startRecording({ withMarkers: true, withAllocations: true, withTicks: true });
+  let rec = yield front.startRecording(
+    { withMarkers: true, withAllocations: true, withTicks: true });
 
   yield waitUntil(() => rec.getAllocations().frames.length);
   yield waitUntil(() => rec.getAllocations().timestamps.length);
@@ -26,10 +28,11 @@ add_task(function* () {
 
   yield front.stopRecording(rec);
 
-  let { frames, timestamps, sizes, sites } = rec.getAllocations();
+  let { timestamps, sizes } = rec.getAllocations();
 
   is(timestamps.length, sizes.length, "we have the same amount of timestamps and sizes");
-  ok(timestamps.every(time => time > 0 && typeof time === "number"), "all timestamps have numeric values");
+  ok(timestamps.every(time => time > 0 && typeof time === "number"),
+    "all timestamps have numeric values");
   ok(sizes.every(n => n > 0 && typeof n === "number"), "all sizes are positive numbers");
 
   yield front.destroy();
diff --git a/devtools/server/tests/browser/browser_perf-profiler-01.js b/devtools/server/tests/browser/browser_perf-profiler-01.js
index 36d200f01143..1d037a3712a6 100644
--- a/devtools/server/tests/browser/browser_perf-profiler-01.js
+++ b/devtools/server/tests/browser/browser_perf-profiler-01.js
@@ -7,12 +7,13 @@
  * a recording is stopped.
  */
 
+"use strict";
+
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
-const { pmmIsProfilerActive, pmmStopProfiler, pmmLoadFrameScripts } = require("devtools/client/performance/test/helpers/profiler-mm-utils");
+const { pmmIsProfilerActive, pmmLoadFrameScripts } = require("devtools/client/performance/test/helpers/profiler-mm-utils");
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "doc_perf.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_perf.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
diff --git a/devtools/server/tests/browser/browser_perf-profiler-02.js b/devtools/server/tests/browser/browser_perf-profiler-02.js
index 29842cef78da..6f48c6a33c13 100644
--- a/devtools/server/tests/browser/browser_perf-profiler-02.js
+++ b/devtools/server/tests/browser/browser_perf-profiler-02.js
@@ -6,8 +6,10 @@
  * is destroyed if there are other consumers using it.
  */
 
+"use strict";
+
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
-const { pmmIsProfilerActive, pmmStopProfiler, pmmLoadFrameScripts } = require("devtools/client/performance/test/helpers/profiler-mm-utils");
+const { pmmIsProfilerActive, pmmLoadFrameScripts } = require("devtools/client/performance/test/helpers/profiler-mm-utils");
 
 add_task(function* () {
   yield addTab(MAIN_DOMAIN + "doc_perf.html");
diff --git a/devtools/server/tests/browser/browser_perf-profiler-03.js b/devtools/server/tests/browser/browser_perf-profiler-03.js
index 28d87fe455bd..831b3d28ead3 100644
--- a/devtools/server/tests/browser/browser_perf-profiler-03.js
+++ b/devtools/server/tests/browser/browser_perf-profiler-03.js
@@ -8,8 +8,10 @@
  * addon was installed and automatically activated the profiler module).
  */
 
+"use strict";
+
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
-const { pmmIsProfilerActive, pmmStartProfiler, pmmStopProfiler, pmmLoadFrameScripts, pmmClearFrameScripts } = require("devtools/client/performance/test/helpers/profiler-mm-utils");
+const { pmmIsProfilerActive, pmmStartProfiler, pmmLoadFrameScripts, pmmClearFrameScripts } = require("devtools/client/performance/test/helpers/profiler-mm-utils");
 
 add_task(function* () {
   // Ensure the profiler is already running when the test starts.
@@ -29,7 +31,7 @@ add_task(function* () {
   let firstFront = PerformanceFront(client, form);
   yield firstFront.connect();
 
-  let recording = yield firstFront.startRecording();
+  yield firstFront.startRecording();
 
   yield addTab(MAIN_DOMAIN + "doc_perf.html");
   let client2 = new DebuggerClient(DebuggerServer.connectPipe());
diff --git a/devtools/server/tests/browser/browser_perf-realtime-markers.js b/devtools/server/tests/browser/browser_perf-realtime-markers.js
index b9eab211cecf..38a8c0db505b 100644
--- a/devtools/server/tests/browser/browser_perf-realtime-markers.js
+++ b/devtools/server/tests/browser/browser_perf-realtime-markers.js
@@ -5,11 +5,12 @@
  * Test functionality of real time markers.
  */
 
+"use strict";
+
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "doc_perf.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_perf.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -34,7 +35,8 @@ add_task(function* () {
 
   front.on("timeline-data", handler);
 
-  let rec = yield front.startRecording({ withMarkers: true, withMemory: true, withTicks: true });
+  let rec = yield front.startRecording(
+    { withMarkers: true, withMemory: true, withTicks: true });
   yield Promise.all(Object.keys(deferreds).map(type => deferreds[type].promise));
   yield front.stopRecording(rec);
   front.off("timeline-data", handler);
@@ -49,15 +51,18 @@ add_task(function* () {
 
   function handler(name, data) {
     if (name === "markers") {
-      if (counters.markers.length >= 1) { return; }
+      if (counters.markers.length >= 1) {
+        return;
+      }
       ok(data.markers[0].start, "received atleast one marker with `start`");
       ok(data.markers[0].end, "received atleast one marker with `end`");
       ok(data.markers[0].name, "received atleast one marker with `name`");
 
       counters.markers.push(data.markers);
-    }
-    else if (name === "memory") {
-      if (counters.memory.length >= 3) { return; }
+    } else if (name === "memory") {
+      if (counters.memory.length >= 3) {
+        return;
+      }
       let { delta, measurement } = data;
       is(typeof delta, "number", "received `delta` in memory event");
       ok(delta > lastMemoryDelta, "received `delta` in memory event");
@@ -65,9 +70,10 @@ add_task(function* () {
 
       counters.memory.push({ delta, measurement });
       lastMemoryDelta = delta;
-    }
-    else if (name === "ticks") {
-      if (counters.ticks.length >= 3) { return; }
+    } else if (name === "ticks") {
+      if (counters.ticks.length >= 3) {
+        return;
+      }
       let { delta, timestamps } = data;
       ok(delta > lastTickDelta, "received `delta` in ticks event");
 
@@ -76,11 +82,9 @@ add_task(function* () {
 
       counters.ticks.push({ delta, timestamps });
       lastTickDelta = delta;
-    }
-    else if (name === "frames") {
+    } else if (name === "frames") {
       // Nothing to do here.
-    }
-    else {
+    } else {
       ok(false, `Received unknown event: ${name}`);
     }
 
diff --git a/devtools/server/tests/browser/browser_perf-recording-actor-01.js b/devtools/server/tests/browser/browser_perf-recording-actor-01.js
index 683493121af5..cb74ee29e08a 100644
--- a/devtools/server/tests/browser/browser_perf-recording-actor-01.js
+++ b/devtools/server/tests/browser/browser_perf-recording-actor-01.js
@@ -6,11 +6,12 @@
  * completed, and rec data.
  */
 
+"use strict";
+
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "doc_perf.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_perf.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -18,7 +19,8 @@ add_task(function* () {
   let front = PerformanceFront(client, form);
   yield front.connect();
 
-  let rec = yield front.startRecording({ withMarkers: true, withTicks: true, withMemory: true });
+  let rec = yield front.startRecording(
+    { withMarkers: true, withTicks: true, withMemory: true });
   ok(rec.isRecording(), "RecordingModel is recording when created");
   yield busyWait(100);
   yield waitUntil(() => rec.getMemory().length);
@@ -43,7 +45,8 @@ add_task(function* () {
     ok(rec.isCompleted(), "recording is completed once it has profile data");
   } else {
     ok(!rec.isCompleted(), "recording is not yet completed on 'recording-stopping'");
-    ok(rec.isFinalizing(), "recording is considered finalizing between 'recording-stopping' and 'recording-stopped'");
+    ok(rec.isFinalizing(),
+      "recording is finalized between 'recording-stopping' and 'recording-stopped'");
   }
 
   yield stopped;
diff --git a/devtools/server/tests/browser/browser_perf-recording-actor-02.js b/devtools/server/tests/browser/browser_perf-recording-actor-02.js
index 8337ad2ef1a6..021bd685ce99 100644
--- a/devtools/server/tests/browser/browser_perf-recording-actor-02.js
+++ b/devtools/server/tests/browser/browser_perf-recording-actor-02.js
@@ -5,14 +5,15 @@
  * Test that buffer status is correctly updated in recording models.
  */
 
+"use strict";
+
 var BUFFER_SIZE = 20000;
 var config = { bufferSize: BUFFER_SIZE };
 
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "doc_perf.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_perf.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -23,8 +24,10 @@ add_task(function* () {
   yield front.setProfilerStatusInterval(10);
   let model = yield front.startRecording(config);
   let stats = yield once(front, "profiler-status");
-  is(stats.totalSize, BUFFER_SIZE, `profiler-status event has totalSize: ${stats.totalSize}`);
-  ok(stats.position < BUFFER_SIZE, `profiler-status event has position: ${stats.position}`);
+  is(stats.totalSize, BUFFER_SIZE,
+    `profiler-status event has totalSize: ${stats.totalSize}`);
+  ok(stats.position < BUFFER_SIZE,
+    `profiler-status event has position: ${stats.position}`);
   ok(stats.generation >= 0, `profiler-status event has generation: ${stats.generation}`);
   ok(stats.isActive, "profiler-status event is isActive");
   is(typeof stats.currentTime, "number", "profiler-status event has currentTime");
@@ -36,7 +39,8 @@ add_task(function* () {
   let checkCount = 0;
   while (lastBufferStatus < 1) {
     let currentBufferStatus = front.getBufferUsageForRecording(model);
-    ok(currentBufferStatus > lastBufferStatus, `buffer is more filled than before: ${currentBufferStatus} > ${lastBufferStatus}`);
+    ok(currentBufferStatus > lastBufferStatus,
+      `buffer is more filled than before: ${currentBufferStatus} > ${lastBufferStatus}`);
     lastBufferStatus = currentBufferStatus;
     checkCount++;
     yield once(front, "profiler-status");
@@ -46,7 +50,8 @@ add_task(function* () {
   is(lastBufferStatus, 1, "buffer usage cannot surpass 100%");
   yield front.stopRecording(model);
 
-  is(front.getBufferUsageForRecording(model), null, "buffer usage should be null when no longer recording.");
+  is(front.getBufferUsageForRecording(model), null,
+    "buffer usage should be null when no longer recording.");
 
   yield front.destroy();
   yield client.close();
diff --git a/devtools/server/tests/browser/browser_perf-samples-01.js b/devtools/server/tests/browser/browser_perf-samples-01.js
index f8f4bf393775..71da7fae10ba 100644
--- a/devtools/server/tests/browser/browser_perf-samples-01.js
+++ b/devtools/server/tests/browser/browser_perf-samples-01.js
@@ -6,13 +6,15 @@
  * normalized before passed to consumers.
  */
 
-const WAIT_TIME = 1000; // ms
+"use strict";
+
+// time in ms
+const WAIT_TIME = 1000;
 
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "doc_perf.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_perf.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -26,7 +28,8 @@ add_task(function* () {
   let firstRecordingStartTime = firstRecording._startTime;
   info("Started profiling at: " + firstRecordingStartTime);
 
-  busyWait(WAIT_TIME); // allow the profiler module to sample some cpu activity
+  // allow the profiler module to sample some cpu activity
+  busyWait(WAIT_TIME);
 
   yield front.stopRecording(firstRecording);
 
@@ -39,7 +42,8 @@ add_task(function* () {
   let secondRecordingStartTime = secondRecording._startTime;
   info("Started profiling at: " + secondRecordingStartTime);
 
-  busyWait(WAIT_TIME); // allow the profiler module to sample more cpu activity
+  // allow the profiler module to sample more cpu activity
+  busyWait(WAIT_TIME);
 
   yield front.stopRecording(secondRecording);
   let secondRecordingProfile = secondRecording.getProfile();
@@ -53,7 +57,9 @@ add_task(function* () {
     "The second recorded sample times were normalized.");
   ok(secondRecordingSamples[0][TIME_SLOT] > 0,
     "The second recorded sample times were normalized correctly.");
-  ok(!secondRecordingSamples.find(e => e[TIME_SLOT] + secondRecordingStartTime <= firstRecording.getDuration()),
+  ok(!secondRecordingSamples.find(
+        e => e[TIME_SLOT] + secondRecordingStartTime <= firstRecording.getDuration()
+    ),
     "There should be no samples from the first recording in the second one, " +
     "even though the total number of frames did not overflow.");
 
diff --git a/devtools/server/tests/browser/browser_perf-samples-02.js b/devtools/server/tests/browser/browser_perf-samples-02.js
index c4d51230d92d..3aad52b6d795 100644
--- a/devtools/server/tests/browser/browser_perf-samples-02.js
+++ b/devtools/server/tests/browser/browser_perf-samples-02.js
@@ -7,13 +7,15 @@
  * devtools/client/performance/modules/logic/tree-model.js will have to be changed.
  */
 
-const WAIT_TIME = 1000; // ms
+"use strict";
+
+// Time in ms
+const WAIT_TIME = 1000;
 
 const { PerformanceFront } = require("devtools/shared/fronts/performance");
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "doc_perf.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "doc_perf.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -22,7 +24,8 @@ add_task(function* () {
   yield front.connect();
 
   let rec = yield front.startRecording();
-  busyWait(WAIT_TIME); // allow the profiler module to sample some cpu activity
+  // allow the profiler module to sample some cpu activity
+  busyWait(WAIT_TIME);
 
   yield front.stopRecording(rec);
   let profile = rec.getProfile();
diff --git a/devtools/server/tests/browser/browser_register_actor.js b/devtools/server/tests/browser/browser_register_actor.js
index 73ee0cedcf6e..6780d3e49f40 100644
--- a/devtools/server/tests/browser/browser_register_actor.js
+++ b/devtools/server/tests/browser/browser_register_actor.js
@@ -1,9 +1,11 @@
+"use strict";
+
 var gClient;
 
 function test() {
   waitForExplicitFinish();
-  var {ActorRegistryFront} = require("devtools/shared/fronts/actor-registry");
-  var actorURL = "chrome://mochitests/content/chrome/devtools/server/tests/mochitest/hello-actor.js";
+  let {ActorRegistryFront} = require("devtools/shared/fronts/actor-registry");
+  let actorURL = "chrome://mochitests/content/chrome/devtools/server/tests/mochitest/hello-actor.js";
 
   if (!DebuggerServer.initialized) {
     DebuggerServer.init();
@@ -13,64 +15,60 @@ function test() {
   gClient = new DebuggerClient(DebuggerServer.connectPipe());
   gClient.connect()
     .then(() => gClient.listTabs())
-    .then(aResponse => {
-
-      var options = {
+    .then(response => {
+      let options = {
         prefix: "helloActor",
         constructor: "HelloActor",
         type: { tab: true }
       };
 
-      var registry = ActorRegistryFront(gClient, aResponse);
+      let registry = ActorRegistryFront(gClient, response);
       registry.registerActor(actorURL, options).then(actorFront => {
-        gClient.listTabs(response => {
-          var tab = response.tabs[response.selected];
+        gClient.listTabs(res => {
+          let tab = res.tabs[res.selected];
           ok(!!tab.helloActor, "Hello actor must exist");
 
           // Make sure actor's state is maintained across listTabs requests.
-          checkActorState(tab.helloActor, () => {
-
-            // Clean up
-            actorFront.unregister().then(() => {
-              gClient.close().then(() => {
-                DebuggerServer.destroy();
-                gClient = null;
-                finish();
-              });
-            });
-          });
+          checkActorState(tab.helloActor, cleanupActor.bind(this, actorFront));
         });
       });
     });
 }
 
-function checkActorState(helloActor, callback) {
-  getCount(helloActor, response => {
-    ok(!response.error, "No error");
-    is(response.count, 1, "The counter must be valid");
-
-    getCount(helloActor, response => {
-      ok(!response.error, "No error");
-      is(response.count, 2, "The counter must be valid");
-
-      gClient.listTabs(response => {
-        var tab = response.tabs[response.selected];
-        is(tab.helloActor, helloActor, "Hello actor must be valid");
-
-        getCount(helloActor, response => {
-          ok(!response.error, "No error");
-          is(response.count, 3, "The counter must be valid");
-
-          callback();
-        });
-      });
+function cleanupActor(actorFront) {
+  // Clean up
+  actorFront.unregister().then(() => {
+    gClient.close().then(() => {
+      DebuggerServer.destroy();
+      gClient = null;
+      finish();
     });
   });
 }
 
 function getCount(actor, callback) {
-  gClient.request({
+  return gClient.request({
     to: actor,
     type: "count"
   }, callback);
 }
+
+var checkActorState = Task.async(function* (helloActor, callback) {
+  let response = yield getCount(helloActor);
+  ok(!response.error, "No error");
+  is(response.count, 1, "The counter must be valid");
+
+  response = yield getCount(helloActor);
+  ok(!response.error, "No error");
+  is(response.count, 2, "The counter must be valid");
+
+  let {tabs, selected} = yield gClient.listTabs();
+  let tab = tabs[selected];
+  is(tab.helloActor, helloActor, "Hello actor must be valid");
+
+  response = yield getCount(helloActor);
+  ok(!response.error, "No error");
+  is(response.count, 3, "The counter must be valid");
+
+  callback();
+});
diff --git a/devtools/server/tests/browser/browser_storage_dynamic_windows.js b/devtools/server/tests/browser/browser_storage_dynamic_windows.js
index a8f791f15086..71ab2adb7697 100644
--- a/devtools/server/tests/browser/browser_storage_dynamic_windows.js
+++ b/devtools/server/tests/browser/browser_storage_dynamic_windows.js
@@ -163,8 +163,10 @@ function testAddIframe(front) {
 
     front.on("stores-update", onStoresUpdate);
 
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
     let iframe = content.document.createElement("iframe");
     iframe.src = ALT_DOMAIN_SECURED + "storage-secured-iframe.html";
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
     content.document.querySelector("body").appendChild(iframe);
   });
 }
@@ -229,12 +231,14 @@ function testRemoveIframe(front) {
 
     front.on("stores-update", onStoresUpdate);
 
-    for (let iframe of content.document.querySelectorAll("iframe")) {
-      if (iframe.src.startsWith("http:")) {
-        iframe.remove();
-        break;
+    ContentTask.spawn(gBrowser.selectedBrowser, {}, () => {
+      for (let iframe of content.document.querySelectorAll("iframe")) {
+        if (iframe.src.startsWith("http:")) {
+          iframe.remove();
+          break;
+        }
       }
-    }
+    });
   });
 }
 
diff --git a/devtools/server/tests/browser/browser_storage_listings.js b/devtools/server/tests/browser/browser_storage_listings.js
index 203e18463829..d5233d0be126 100644
--- a/devtools/server/tests/browser/browser_storage_listings.js
+++ b/devtools/server/tests/browser/browser_storage_listings.js
@@ -228,7 +228,7 @@ const IDBValues = {
         },
       ]
     },
-    "http://sectest1.example.org" : {},
+    "http://sectest1.example.org": {},
     "https://sectest1.example.org": {
       "idb-s1 (default)": [
         {
@@ -297,7 +297,7 @@ const IDBValues = {
       ],
       "idb2 (default)#obj3": []
     },
-    "http://sectest1.example.org" : {},
+    "http://sectest1.example.org": {},
     "https://sectest1.example.org": {
       "idb-s1 (default)#obj-s1": [
         {
@@ -331,13 +331,6 @@ const IDBValues = {
   }
 };
 
-function finishTests(client) {
-
-  let closeConnection = () => {
-
-  };
-}
-
 function* testStores(data) {
   ok(data.cookies, "Cookies storage actor is present");
   ok(data.localStorage, "Local Storage storage actor is present");
@@ -514,8 +507,8 @@ var testIndexedDBs = Task.async(function* (index, hosts, indexedDBActor) {
   yield testIndexedDBs(++index, hosts, indexedDBActor);
 });
 
-var testObjectStores = Task.async(function* (index, hosts, indexedDBActor) {
-  let host = Object.keys(hosts)[index];
+var testObjectStores = Task.async(function* (ix, hosts, indexedDBActor) {
+  let host = Object.keys(hosts)[ix];
   let matchItems = (data, db) => {
     is(data.total, IDBValues.objectStoreDetails[host][db].length,
        "Number of object stores in host " + host + " matches");
@@ -559,10 +552,10 @@ var testObjectStores = Task.async(function* (index, hosts, indexedDBActor) {
       yield indexedDBActor.getStoreObjects(host, [JSON.stringify(objName)])
     ), objName[0]);
   }
-  if (index == Object.keys(hosts).length - 1) {
+  if (ix == Object.keys(hosts).length - 1) {
     return;
   }
-  yield testObjectStores(++index, hosts, indexedDBActor);
+  yield testObjectStores(++ix, hosts, indexedDBActor);
 });
 
 var testIDBEntries = Task.async(function* (index, hosts, indexedDBActor) {
diff --git a/devtools/server/tests/browser/browser_storage_updates.js b/devtools/server/tests/browser/browser_storage_updates.js
index 4a160478717c..8e6cc9587d3e 100644
--- a/devtools/server/tests/browser/browser_storage_updates.js
+++ b/devtools/server/tests/browser/browser_storage_updates.js
@@ -5,11 +5,6 @@
 "use strict";
 
 const {StorageFront} = require("devtools/shared/fronts/storage");
-const beforeReload = {
-  cookies: ["http://test1.example.org", "https://sectest1.example.org"],
-  localStorage: ["http://test1.example.org", "http://sectest1.example.org"],
-  sessionStorage: ["http://test1.example.org", "http://sectest1.example.org"],
-};
 
 const TESTS = [
   // index 0
@@ -303,6 +298,7 @@ function* finishTests(client) {
 
 add_task(function* () {
   let browser = yield addTab(MAIN_DOMAIN + "storage-updates.html");
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
   let doc = browser.contentDocument;
 
   initDebuggerServer();
diff --git a/devtools/server/tests/browser/browser_stylesheets_nested-iframes.js b/devtools/server/tests/browser/browser_stylesheets_nested-iframes.js
index c382b6ce8a33..119239f2635b 100644
--- a/devtools/server/tests/browser/browser_stylesheets_nested-iframes.js
+++ b/devtools/server/tests/browser/browser_stylesheets_nested-iframes.js
@@ -10,8 +10,7 @@
 const {StyleSheetsFront} = require("devtools/shared/fronts/stylesheets");
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "stylesheets-nested-iframes.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "stylesheets-nested-iframes.html");
 
   info("Initialising the debugger server and client.");
   initDebuggerServer();
diff --git a/devtools/server/tests/browser/browser_timeline.js b/devtools/server/tests/browser/browser_timeline.js
index 1e57934477cf..8d2a7a34bb08 100644
--- a/devtools/server/tests/browser/browser_timeline.js
+++ b/devtools/server/tests/browser/browser_timeline.js
@@ -13,8 +13,7 @@
 const {TimelineFront} = require("devtools/shared/fronts/timeline");
 
 add_task(function* () {
-  let browser = yield addTab("data:text/html;charset=utf-8,mop");
-  let doc = browser.contentDocument;
+  yield addTab("data:text/html;charset=utf-8,mop");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -27,7 +26,10 @@ add_task(function* () {
   ok(!isActive, "The TimelineFront is not initially recording");
 
   info("Flush any pending reflows");
-  let forceSyncReflow = doc.body.innerHeight;
+  ContentTask.spawn(gBrowser.selectedBrowser, {}, () => {
+    // forceSyncReflow
+    content.document.body.innerHeight;
+  });
 
   info("Start recording");
   yield front.start({ withMarkers: true });
@@ -37,18 +39,25 @@ add_task(function* () {
 
   info("Change some style on the page to cause style/reflow/paint");
   let onMarkers = once(front, "markers");
-  doc.body.style.padding = "10px";
+  ContentTask.spawn(gBrowser.selectedBrowser, {}, () => {
+    content.document.body.style.padding = "10px";
+  });
   let markers = yield onMarkers;
 
   ok(true, "The markers event was fired");
   ok(markers.length > 0, "Markers were returned");
 
   info("Flush pending reflows again");
-  forceSyncReflow = doc.body.innerHeight;
+  ContentTask.spawn(gBrowser.selectedBrowser, {}, () => {
+    // forceSyncReflow
+    content.document.body.innerHeight;
+  });
 
   info("Change some style on the page to cause style/paint");
   onMarkers = once(front, "markers");
-  doc.body.style.backgroundColor = "red";
+  ContentTask.spawn(gBrowser.selectedBrowser, {}, () => {
+    content.document.body.style.backgroundColor = "red";
+  });
   markers = yield onMarkers;
 
   ok(markers.length > 0, "markers were returned");
diff --git a/devtools/server/tests/browser/browser_timeline_actors.js b/devtools/server/tests/browser/browser_timeline_actors.js
index a902775fae01..35c8db92dafc 100644
--- a/devtools/server/tests/browser/browser_timeline_actors.js
+++ b/devtools/server/tests/browser/browser_timeline_actors.js
@@ -10,8 +10,7 @@
 const {TimelineFront} = require("devtools/shared/fronts/timeline");
 
 add_task(function* () {
-  let browser = yield addTab("data:text/html;charset=utf-8,mop");
-  let doc = browser.contentDocument;
+  yield addTab("data:text/html;charset=utf-8,mop");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
diff --git a/devtools/server/tests/browser/browser_timeline_iframes.js b/devtools/server/tests/browser/browser_timeline_iframes.js
index 60728873f9eb..8b9c0c8c4dd0 100644
--- a/devtools/server/tests/browser/browser_timeline_iframes.js
+++ b/devtools/server/tests/browser/browser_timeline_iframes.js
@@ -10,8 +10,7 @@
 const {TimelineFront} = require("devtools/shared/fronts/timeline");
 
 add_task(function* () {
-  let browser = yield addTab(MAIN_DOMAIN + "timeline-iframe-parent.html");
-  let doc = browser.contentDocument;
+  yield addTab(MAIN_DOMAIN + "timeline-iframe-parent.html");
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
@@ -24,7 +23,8 @@ add_task(function* () {
   // Check that we get markers for a few iterations of the timer that runs in
   // the child frame.
   for (let i = 0; i < 3; i++) {
-    yield wait(300); // That's the time the child frame waits before changing styles.
+    // That's the time the child frame waits before changing styles.
+    yield wait(300);
     let markers = yield once(front, "markers");
     ok(markers.length, "Markers were received for operations in the child frame");
   }
diff --git a/devtools/server/tests/browser/director-script-target.html b/devtools/server/tests/browser/director-script-target.html
index 0b0b56d64022..ffcdd80a183b 100644
--- a/devtools/server/tests/browser/director-script-target.html
+++ b/devtools/server/tests/browser/director-script-target.html
@@ -1,11 +1,13 @@
 <html>
   <head>
     <script>
-      // change the eval function to ensure the window object in the debug-script is correctly wrapped
+      /* exported globalAccessibleVar */
+      "use strict";
+      // change the eval function to ensure the window object
+      // in the debug-script is correctly wrapped
       window.eval = function () {
         return "unsecure-eval-called";
       };
-
       var globalAccessibleVar = "global-value";
     </script>
   </head>
diff --git a/devtools/server/tests/browser/doc_allocations.html b/devtools/server/tests/browser/doc_allocations.html
index 314c40cac259..eca7849dc8bf 100644
--- a/devtools/server/tests/browser/doc_allocations.html
+++ b/devtools/server/tests/browser/doc_allocations.html
@@ -5,11 +5,13 @@
 </head>
 <body>
 <script>
+"use strict";
+
 window.allocs = [];
-window.onload = function() {
+window.onload = function () {
   function allocator() {
-    for (var i = 0; i < 1000; i++) {
-      window.allocs.push(new Object);
+    for (let i = 0; i < 1000; i++) {
+      window.allocs.push(new Object());
     }
   }
 
diff --git a/devtools/server/tests/browser/doc_force_cc.html b/devtools/server/tests/browser/doc_force_cc.html
index d5868bd6b7c7..abf1cfd2f5db 100644
--- a/devtools/server/tests/browser/doc_force_cc.html
+++ b/devtools/server/tests/browser/doc_force_cc.html
@@ -10,6 +10,8 @@
 
   <body>
     <script type="text/javascript">
+    "use strict";
+
     window.test = function () {
       document.body.expando1 = { cycle: document.body };
       SpecialPowers.Cu.forceCC();
diff --git a/devtools/server/tests/browser/doc_force_gc.html b/devtools/server/tests/browser/doc_force_gc.html
index f8b6175336c9..54d40b5ecb34 100644
--- a/devtools/server/tests/browser/doc_force_gc.html
+++ b/devtools/server/tests/browser/doc_force_gc.html
@@ -10,12 +10,15 @@
 
   <body>
     <script type="text/javascript">
+    "use strict";
+
     var x = 1;
     window.test = function () {
       SpecialPowers.Cu.forceGC();
       document.body.style.borderTop = x + "px solid red";
-      x = 1^x;
-      document.body.innerHeight; // flush pending reflows
+      x = 1 ^ x;
+      // flush pending reflows
+      document.body.innerHeight;
 
       // Prevent this script from being garbage collected.
       setTimeout(window.test, 100);
diff --git a/devtools/server/tests/browser/doc_perf.html b/devtools/server/tests/browser/doc_perf.html
index 1da36328be46..3d18f902862d 100644
--- a/devtools/server/tests/browser/doc_perf.html
+++ b/devtools/server/tests/browser/doc_perf.html
@@ -10,11 +10,14 @@
 
   <body>
     <script type="text/javascript">
+      "use strict";
+
       var x = 1;
       function test() {
         document.body.style.borderTop = x + "px solid red";
-        x = 1^x;
-        document.body.innerHeight; // flush pending reflows
+        x = 1 ^ x;
+        // flush pending reflows
+        document.body.innerHeight;
       }
 
       // Prevent this script from being garbage collected.
diff --git a/devtools/server/tests/browser/navigate-first.html b/devtools/server/tests/browser/navigate-first.html
index 829372427925..444852f03ad4 100644
--- a/devtools/server/tests/browser/navigate-first.html
+++ b/devtools/server/tests/browser/navigate-first.html
@@ -6,9 +6,9 @@
 <body>
 First
 <script>
-
-window.onbeforeunload=function(e){
-  e.returnValue="?";
+"use strict";
+window.onbeforeunload = function (e) {
+  e.returnValue = "?";
 };
 </script>
 </body>
diff --git a/devtools/server/tests/browser/storage-helpers.js b/devtools/server/tests/browser/storage-helpers.js
index 1c4f37705b31..5c8283352fec 100644
--- a/devtools/server/tests/browser/storage-helpers.js
+++ b/devtools/server/tests/browser/storage-helpers.js
@@ -1,6 +1,8 @@
 /* Any copyright is dedicated to the Public Domain.
    http://creativecommons.org/publicdomain/zero/1.0/ */
 
+/* exported openTabAndSetupStorage, clearStorage */
+
 "use strict";
 
 /**
diff --git a/devtools/server/tests/browser/storage-unsecured-iframe.html b/devtools/server/tests/browser/storage-unsecured-iframe.html
index d339fb7f2551..4cf3008c4597 100644
--- a/devtools/server/tests/browser/storage-unsecured-iframe.html
+++ b/devtools/server/tests/browser/storage-unsecured-iframe.html
@@ -8,6 +8,7 @@ Iframe for testing multiple host detetion in storage actor
 </head>
 <body>
 <script>
+"use strict";
 
 document.cookie = "uc1=foobar; domain=.example.org; path=/; secure=true";
 localStorage.setItem("iframe-u-ls1", "foobar");
@@ -15,12 +16,13 @@ sessionStorage.setItem("iframe-u-ss1", "foobar1");
 sessionStorage.setItem("iframe-u-ss2", "foobar2");
 console.log("added cookies and stuff from unsecured iframe");
 
-window.clear = function*() {
+window.clear = function* () {
   document.cookie = "uc1=; expires=Thu, 01 Jan 1970 00:00:00 GMT";
   localStorage.clear();
   sessionStorage.clear();
   console.log("removed cookies and stuff from unsecured iframe");
-}
+};
+
 </script>
 </body>
 </html>
diff --git a/devtools/server/tests/browser/timeline-iframe-child.html b/devtools/server/tests/browser/timeline-iframe-child.html
index 5385c64859eb..038561dffc9d 100644
--- a/devtools/server/tests/browser/timeline-iframe-child.html
+++ b/devtools/server/tests/browser/timeline-iframe-child.html
@@ -7,12 +7,13 @@
 <body>
   <h1>Child frame</h1>
   <script>
+    "use strict";
     var h1 = document.querySelector("h1");
-    setInterval(function() {
-      h1.style.backgroundColor = "rgb(" + ((Math.random()*255)|0) + "," +
-                                          ((Math.random()*255)|0) + "," +
-                                          ((Math.random()*255)|0) +")";
-      h1.style.width = ((Math.random()*500)|0) + "px";
+    setInterval(function () {
+      h1.style.backgroundColor = "rgb(" + ((Math.random() * 255)|0) + "," +
+                                          ((Math.random() * 255)|0) + "," +
+                                          ((Math.random() * 255)|0) + ")";
+      h1.style.width = ((Math.random() * 500)|0) + "px";
     }, 300);
   </script>
 </body>