diff --git a/.eslintignore b/.eslintignore
index fefc7a1f8a69..b6737bf89bf5 100644
--- a/.eslintignore
+++ b/.eslintignore
@@ -208,7 +208,7 @@ dom/security/test/sri/**
 dom/serviceworkers/**
 dom/smil/**
 dom/svg/**
-dom/tests/browser/**
+
 dom/tests/html/**
 dom/tests/mochitest/**
 dom/u2f/**
diff --git a/dom/tests/browser/browser_ConsoleAPITests.js b/dom/tests/browser/browser_ConsoleAPITests.js
index 23caf4541892..bd769360e74f 100644
--- a/dom/tests/browser/browser_ConsoleAPITests.js
+++ b/dom/tests/browser/browser_ConsoleAPITests.js
@@ -101,6 +101,8 @@ async function consoleAPISanityTest(browser) {
   });
 }
 
+// These globals are all defined in spawnWithObserver in a sub-process.
+/* global gWindow, gArgs:true, gLevel:true, gStyle:true, expect, resolve */
 function testConsoleData(aMessageObject) {
   let messageWindow = Services.wm.getOuterWindowWithId(aMessageObject.ID);
   is(messageWindow, gWindow, "found correct window by window ID");
diff --git a/dom/tests/browser/browser_allocateGigabyte.js b/dom/tests/browser/browser_allocateGigabyte.js
index 4ec45b2f6056..9cbb1ff54250 100644
--- a/dom/tests/browser/browser_allocateGigabyte.js
+++ b/dom/tests/browser/browser_allocateGigabyte.js
@@ -5,7 +5,7 @@
 const TEST_URI = "http://example.com/browser/dom/tests/browser/test_largeAllocation.html";
 
 function expectProcessCreated() {
-  let os = Cc["@mozilla.org/observer-service;1"].getService(Ci.nsIObserverService);
+  let os = Services.obs;
   return new Promise(resolve => {
     let topic = "ipc:content-created";
     function observer() {
diff --git a/dom/tests/browser/browser_beforeunload_between_chrome_content.js b/dom/tests/browser/browser_beforeunload_between_chrome_content.js
index 6c26eacc4b61..83cae3e421ce 100644
--- a/dom/tests/browser/browser_beforeunload_between_chrome_content.js
+++ b/dom/tests/browser/browser_beforeunload_between_chrome_content.js
@@ -41,6 +41,7 @@ SpecialPowers.pushPrefEnv(
  * Test navigation from a content page to a chrome page. Also check that only
  * one beforeunload event is fired.
  */
+/* global messageManager */
 add_task(async function() {
   let beforeUnloadCount = 0;
   messageManager.addMessageListener("Test:OnBeforeUnloadReceived", function() {
diff --git a/dom/tests/browser/browser_bug396843.js b/dom/tests/browser/browser_bug396843.js
index 8b4b2de41f14..e7886967e5a3 100644
--- a/dom/tests/browser/browser_bug396843.js
+++ b/dom/tests/browser/browser_bug396843.js
@@ -273,6 +273,7 @@ async function newTabTest(location) {
         async function(browser) {
           await ContentTask.spawn(browser, { location, testInDocument_: testInDocument.toSource() },
             async function({ location, testInDocument_ }) {
+              // eslint-disable-next-line no-eval
               let testInDocument = eval(`(() => (${testInDocument_}))()`);
               testInDocument(content.document, location);
             });
diff --git a/dom/tests/browser/browser_focus_steal_from_chrome.js b/dom/tests/browser/browser_focus_steal_from_chrome.js
index 3a8612bb2e4a..9bb22932ab8e 100644
--- a/dom/tests/browser/browser_focus_steal_from_chrome.js
+++ b/dom/tests/browser/browser_focus_steal_from_chrome.js
@@ -50,6 +50,7 @@ add_task(async function() {
             function f() {
               let e = content.document.activeElement;
               if (e.tagName != test.tagName) {
+                // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
                 setTimeout(f, 10);
               } else {
                 is(Services.focus.focusedElement, e,
@@ -68,6 +69,7 @@ add_task(async function() {
             function f() {
               let e = content.document.activeElement;
               if (e.tagName != test.tagName) {
+                // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
                 setTimeout(f, 10);
               } else {
                 isnot(Services.focus.focusedElement, e,
@@ -110,6 +112,7 @@ add_task(async function() {
             function f() {
               let e = content.document.activeElement;
               if (e.tagName != test.tagName) {
+                // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
                 setTimeout(f, 10);
               } else {
                 isnot(Services.focus.focusedElement, e,
@@ -128,6 +131,7 @@ add_task(async function() {
             function f() {
               let e = content.document.activeElement;
               if (e.tagName != test.tagName) {
+                // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
                 setTimeout(f, 10);
               } else {
                 isnot(Services.focus.focusedElement, e,
diff --git a/dom/tests/browser/browser_focus_steal_from_chrome_during_mousedown.js b/dom/tests/browser/browser_focus_steal_from_chrome_during_mousedown.js
index f05b1237b8b9..1bf56989596b 100644
--- a/dom/tests/browser/browser_focus_steal_from_chrome_during_mousedown.js
+++ b/dom/tests/browser/browser_focus_steal_from_chrome_during_mousedown.js
@@ -13,8 +13,7 @@ add_task(async function test() {
 
   let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, kTestURI);
 
-  let fm = Cc["@mozilla.org/focus-manager;1"].
-        getService(Ci.nsIFocusManager);
+  let fm = Services.focus;
 
   for (var button = 0; button < 3; button++) {
     // Set focus to a chrome element before synthesizing a mouse down event.
@@ -31,8 +30,7 @@ add_task(async function test() {
        "Failed to move focus away from search bar: button=" + button);
 
     await ContentTask.spawn(tab.linkedBrowser, button, async function(button) {
-      let fm = Cc["@mozilla.org/focus-manager;1"].
-          getService(Ci.nsIFocusManager);
+      let fm = Services.focus;
 
       let attempts = 10;
       await new Promise(resolve => {
diff --git a/dom/tests/browser/browser_keypressTelemetry.js b/dom/tests/browser/browser_keypressTelemetry.js
index 8d3a30b111d7..ee2ab16ba199 100644
--- a/dom/tests/browser/browser_keypressTelemetry.js
+++ b/dom/tests/browser/browser_keypressTelemetry.js
@@ -8,23 +8,21 @@ var PaintListener = {};
 Services.scriptloader.loadSubScript("chrome://mochikit/content/tests/SimpleTest/EventUtils.js", EventUtils);
 const { ContentTaskUtils } = ChromeUtils.import("resource://testing-common/ContentTaskUtils.jsm", {});
 
-function getRecordedKeypressCount()
-{
+function getRecordedKeypressCount() {
     let snapshot = Services.telemetry.getSnapshotForHistograms("main", false);
 
     var totalCount = 0;
     for (var prop in snapshot) {
-        if (snapshot[prop]["KEYPRESS_PRESENT_LATENCY"]) {
+        if (snapshot[prop].KEYPRESS_PRESENT_LATENCY) {
             dump("found snapshot");
-            totalCount += Object.values(snapshot[prop]["KEYPRESS_PRESENT_LATENCY"].values).reduce((a, b) => a + b, 0);
+            totalCount += Object.values(snapshot[prop].KEYPRESS_PRESENT_LATENCY.values).reduce((a, b) => a + b, 0);
         }
     }
 
     return totalCount;
 }
 
-add_task(async function()
-{
+add_task(async function() {
     await SpecialPowers.pushPrefEnv({set: [["toolkit.telemetry.ipcBatchTimeout", 10]]});
     let histogram = Services.telemetry.getHistogramById("KEYPRESS_PRESENT_LATENCY");
     histogram.clear();
@@ -33,17 +31,17 @@ add_task(async function()
 
     gURLBar.focus();
     await SimpleTest.promiseFocus(window);
-    EventUtils.sendChar('x');
+    EventUtils.sendChar("x");
 
-    await ContentTaskUtils.waitForCondition(() => { return getRecordedKeypressCount() > 0; }, "waiting for telemetry", 200, 600)
+    await ContentTaskUtils.waitForCondition(() => { return getRecordedKeypressCount() > 0; }, "waiting for telemetry", 200, 600);
     let result = getRecordedKeypressCount();
     ok(result == 1, "One keypress recorded");
 
     gURLBar.focus();
     await SimpleTest.promiseFocus(window);
-    EventUtils.sendChar('x');
+    EventUtils.sendChar("x");
 
-    await ContentTaskUtils.waitForCondition(() => { return getRecordedKeypressCount() > 1; }, "waiting for telemetry", 200, 600)
+    await ContentTaskUtils.waitForCondition(() => { return getRecordedKeypressCount() > 1; }, "waiting for telemetry", 200, 600);
     result = getRecordedKeypressCount();
     ok(result == 2, "Two keypresses recorded");
 });
diff --git a/dom/tests/browser/browser_largeAllocation_non_win32.js b/dom/tests/browser/browser_largeAllocation_non_win32.js
index a2d4addbc479..61747a5af946 100644
--- a/dom/tests/browser/browser_largeAllocation_non_win32.js
+++ b/dom/tests/browser/browser_largeAllocation_non_win32.js
@@ -1,4 +1,5 @@
 let testDir = gTestPath.substr(0, gTestPath.lastIndexOf("/"));
+/* import-globals-from helper_largeAllocation.js */
 Services.scriptloader.loadSubScript(testDir + "/helper_largeAllocation.js", this);
 
 // Force-enabling the Large-Allocation header
diff --git a/dom/tests/browser/browser_largeAllocation_win32.js b/dom/tests/browser/browser_largeAllocation_win32.js
index f002a5c9aaab..03f8cd42778a 100644
--- a/dom/tests/browser/browser_largeAllocation_win32.js
+++ b/dom/tests/browser/browser_largeAllocation_win32.js
@@ -12,7 +12,7 @@ add_task(async function() {
       ["dom.ipc.processCount.webLargeAllocation", 20],
     ],
   });
-
+  /* global largeAllocSuccessTests */
   await largeAllocSuccessTests();
 });
 
diff --git a/dom/tests/browser/browser_localStorage_e10s.js b/dom/tests/browser/browser_localStorage_e10s.js
index a1ac7e36e8ad..5281ea1495aa 100644
--- a/dom/tests/browser/browser_localStorage_e10s.js
+++ b/dom/tests/browser/browser_localStorage_e10s.js
@@ -5,74 +5,6 @@ const HELPER_PAGE_ORIGIN = "http://example.com/";
 let testDir = gTestPath.substr(0, gTestPath.lastIndexOf("/"));
 Services.scriptloader.loadSubScript(testDir + "/helper_localStorage_e10s.js",
                                     this);
-// Simple tab wrapper abstracting our messaging mechanism;
-class KnownTab {
-  constructor(name, tab) {
-    this.name = name;
-    this.tab = tab;
-  }
-
-  cleanup() {
-    this.tab = null;
-  }
-}
-
-// Simple data structure class to help us track opened tabs and their pids.
-class KnownTabs {
-  constructor() {
-    this.byPid = new Map();
-    this.byName = new Map();
-  }
-
-  cleanup() {
-    this.byPid = null;
-    this.byName = null;
-  }
-}
-
-/**
- * Open our helper page in a tab in its own content process, asserting that it
- * really is in its own process.  We initially load and wait for about:blank to
- * load, and only then loadURI to our actual page.  This is to ensure that
- * LocalStorageManager has had an opportunity to be created and populate
- * mOriginsHavingData.
- *
- * (nsGlobalWindow will reliably create LocalStorageManager as a side-effect of
- * the unconditional call to nsGlobalWindow::PreloadLocalStorage.  This will
- * reliably create the StorageDBChild instance, and its corresponding
- * StorageDBParent will send the set of origins when it is constructed.)
- */
-async function openTestTabInOwnProcess(name, knownTabs) {
-  let realUrl = HELPER_PAGE_URL + "?" + encodeURIComponent(name);
-  // Load and wait for about:blank.
-  let tab = await BrowserTestUtils.openNewForegroundTab({
-    gBrowser, opening: "about:blank", forceNewProcess: true,
-  });
-  let pid = tab.linkedBrowser.frameLoader.tabParent.osPid;
-  ok(!knownTabs.byName.has(name), "tab needs its own name: " + name);
-  ok(!knownTabs.byPid.has(pid), "tab needs to be in its own process: " + pid);
-
-  let knownTab = new KnownTab(name, tab);
-  knownTabs.byPid.set(pid, knownTab);
-  knownTabs.byName.set(name, knownTab);
-
-  // Now trigger the actual load of our page.
-  BrowserTestUtils.loadURI(tab.linkedBrowser, realUrl);
-  await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
-  is(tab.linkedBrowser.frameLoader.tabParent.osPid, pid, "still same pid");
-  return knownTab;
-}
-
-/**
- * Close all the tabs we opened.
- */
-async function cleanupTabs(knownTabs) {
-  for (let knownTab of knownTabs.byName.values()) {
-    BrowserTestUtils.removeTab(knownTab.tab);
-    knownTab.cleanup();
-  }
-  knownTabs.cleanup();
-}
 
 /**
  * Wait for a LocalStorage flush to occur.  This notification can occur as a
@@ -81,6 +13,9 @@ async function cleanupTabs(knownTabs) {
  * - InsertDBOp seeing a preload op for an origin with outstanding changes.
  * - Us generating a "domstorage-test-flush-force" observer notification.
  */
+
+ /* import-globals-from helper_localStorage_e10s.js */
+
 function waitForLocalStorageFlush() {
   if (Services.lsm.nextGenLocalStorageEnabled) {
     return new Promise(resolve => executeSoon(resolve));
diff --git a/dom/tests/browser/browser_localStorage_snapshotting_e10s.js b/dom/tests/browser/browser_localStorage_snapshotting_e10s.js
index c1a6ac28ffa4..11a819ca8b99 100644
--- a/dom/tests/browser/browser_localStorage_snapshotting_e10s.js
+++ b/dom/tests/browser/browser_localStorage_snapshotting_e10s.js
@@ -2,6 +2,8 @@ const HELPER_PAGE_URL =
   "http://example.com/browser/dom/tests/browser/page_localstorage_snapshotting_e10s.html";
 const HELPER_PAGE_ORIGIN = "http://example.com/";
 
+/* import-globals-from helper_localStorage_e10s.js */
+
 let testDir = gTestPath.substr(0, gTestPath.lastIndexOf("/"));
 Services.scriptloader.loadSubScript(testDir + "/helper_localStorage_e10s.js",
                                     this);
@@ -94,7 +96,7 @@ add_task(async function() {
       // Enable LocalStorage's testing API so we can explicitly create
       // snapshots when needed.
       ["dom.storage.testing", true],
-    ]
+    ],
   });
 
   // Ensure that there is no localstorage data by forcing the origin to be
@@ -120,7 +122,7 @@ add_task(async function() {
     ["key5", "initial5"],
     ["key6", "initial6"],
     ["key7", "initial7"],
-    ["key8", "initial8"]
+    ["key8", "initial8"],
   ];
 
   const initialState = {
@@ -130,11 +132,10 @@ add_task(async function() {
     key5: "initial5",
     key6: "initial6",
     key7: "initial7",
-    key8: "initial8"
+    key8: "initial8",
   };
 
-  function getPartialPrefill()
-  {
+  function getPartialPrefill() {
     let size = 0;
     let entries = Object.entries(initialState);
     for (let i = 0; i < entries.length / 2; i++) {
@@ -147,7 +148,7 @@ add_task(async function() {
   const prefillValues = [
     0,                   // no prefill
     getPartialPrefill(), // partial prefill
-    -1                   // full prefill
+    -1,                  // full prefill
   ];
 
   for (let prefillValue of prefillValues) {
@@ -155,8 +156,8 @@ add_task(async function() {
 
     await SpecialPowers.pushPrefEnv({
       set: [
-        ["dom.storage.snapshot_prefill", prefillValue]
-      ]
+        ["dom.storage.snapshot_prefill", prefillValue],
+      ],
     });
 
     info("Stage 1");
@@ -171,7 +172,7 @@ add_task(async function() {
       ["key6", "setRemove16"],
       ["key7", "setRemove17"],
       ["key8", null],
-      ["key9", "setRemove19"]
+      ["key9", "setRemove19"],
     ];
 
     const setRemoveState1 = {
@@ -182,7 +183,7 @@ add_task(async function() {
       key5: "setRemove15",
       key6: "setRemove16",
       key7: "setRemove17",
-      key9: "setRemove19"
+      key9: "setRemove19",
     };
 
     const setRemoveMutations2 = [
@@ -195,7 +196,7 @@ add_task(async function() {
       ["key6", "setRemove26"],
       ["key7", null],
       ["key8", "setRemove28"],
-      ["key9", "setRemove29"]
+      ["key9", "setRemove29"],
     ];
 
     const setRemoveState2 = {
@@ -206,7 +207,7 @@ add_task(async function() {
       key5: "setRemove25",
       key6: "setRemove26",
       key8: "setRemove28",
-      key9: "setRemove29"
+      key9: "setRemove29",
     };
 
     // Apply initial mutations using an explicit snapshot. The explicit
@@ -259,7 +260,7 @@ add_task(async function() {
     const setRemoveClearMutations1 = [
       ["key0", "setRemoveClear10"],
       ["key1", null],
-      [null, null]
+      [null, null],
     ];
 
     const setRemoveClearState1 = {
@@ -268,7 +269,7 @@ add_task(async function() {
     const setRemoveClearMutations2 = [
       ["key8", null],
       ["key9", "setRemoveClear29"],
-      [null, null]
+      [null, null],
     ];
 
     const setRemoveClearState2 = {
@@ -318,7 +319,7 @@ add_task(async function() {
       ["key5", "initial5"],
       ["key3", "initial3"],
       ["key2", "initial2"],
-      ["key1", "initial1"]
+      ["key1", "initial1"],
     ];
 
     // Apply initial mutations using an explicit snapshot. The explicit
diff --git a/dom/tests/browser/browser_test_focus_after_modal_state.js b/dom/tests/browser/browser_test_focus_after_modal_state.js
index 5ca38a520cee..4a2982af24f8 100644
--- a/dom/tests/browser/browser_test_focus_after_modal_state.js
+++ b/dom/tests/browser/browser_test_focus_after_modal_state.js
@@ -12,7 +12,7 @@ function awaitAndClosePrompt() {
     Services.obs.addObserver(onDialogShown, "tabmodal-dialog-loaded");
   });
 }
-
+/* global messageManager */
 let lastMessageReceived = "";
 function waitForMessage(message) {
   return new Promise((resolve, reject) => {
diff --git a/dom/tests/browser/browser_test_new_window_from_content.js b/dom/tests/browser/browser_test_new_window_from_content.js
index b7c526ad53c9..cd1cf47f5586 100644
--- a/dom/tests/browser/browser_test_new_window_from_content.js
+++ b/dom/tests/browser/browser_test_new_window_from_content.js
@@ -115,7 +115,6 @@ function prepareForResult(aBrowser, aExpectation) {
         await BrowserTestUtils.loadURI(aBrowser, kContentDoc);
         await BrowserTestUtils.browserLoaded(aBrowser);
       })();
-      break;
     case kNewWin:
       return (async function() {
         let newWin = await BrowserTestUtils.waitForNewWindow({url: expectedSpec});
@@ -123,7 +122,6 @@ function prepareForResult(aBrowser, aExpectation) {
         is(newBrowser.currentURI.spec, expectedSpec, "Should be at dummy.html");
         await BrowserTestUtils.closeWindow(newWin);
       })();
-      break;
     case kNewTab:
       return (async function() {
         let newTab = await BrowserTestUtils.waitForNewTab(gBrowser);
@@ -131,13 +129,10 @@ function prepareForResult(aBrowser, aExpectation) {
            "Should be at dummy.html");
         BrowserTestUtils.removeTab(newTab);
       })();
-      break;
     default:
       ok(false, "prepareForResult can't handle an expectation of " + aExpectation);
-      return;
+      return Promise.resolve();
   }
-
-  return deferred.promise;
 }
 
 /**
diff --git a/dom/tests/browser/browser_test_performance_metrics_off.js b/dom/tests/browser/browser_test_performance_metrics_off.js
index 56bcd9bfacb8..904795895885 100644
--- a/dom/tests/browser/browser_test_performance_metrics_off.js
+++ b/dom/tests/browser/browser_test_performance_metrics_off.js
@@ -11,7 +11,7 @@ add_task(async function testNotActivated() {
   // we call requestPerformanceMetrics()
   let failed = false;
   try {
-    let promise = await ChromeUtils.requestPerformanceMetrics();
+    await ChromeUtils.requestPerformanceMetrics();
   } catch (e) {
     failed = true;
   }
diff --git a/dom/tests/browser/browser_xhr_sandbox.js b/dom/tests/browser/browser_xhr_sandbox.js
index 0bdd40207e0f..df950d53665d 100644
--- a/dom/tests/browser/browser_xhr_sandbox.js
+++ b/dom/tests/browser/browser_xhr_sandbox.js
@@ -23,8 +23,7 @@ var sandboxCode = (function() {
 
 function test() {
   waitForExplicitFinish();
-  let appShell = Cc["@mozilla.org/appshell/appShellService;1"]
-                  .getService(Ci.nsIAppShellService);
+  let appShell = Services.appShell;
   let doc = appShell.hiddenDOMWindow.document;
   let frame = doc.createElement("iframe");
   frame.setAttribute("type", "content");
diff --git a/dom/tests/browser/helper_largeAllocation.js b/dom/tests/browser/helper_largeAllocation.js
index 456b47bcb430..093a36743926 100644
--- a/dom/tests/browser/helper_largeAllocation.js
+++ b/dom/tests/browser/helper_largeAllocation.js
@@ -6,7 +6,7 @@ const TEST_URI = "http://example.com/browser/dom/tests/browser/test_largeAllocat
 const TEST_URI_2 = "http://example.com/browser/dom/tests/browser/test_largeAllocation2.html";
 
 function expectProcessCreated() {
-  let os = Cc["@mozilla.org/observer-service;1"].getService(Ci.nsIObserverService);
+  let os = Services.obs;
   let kill; // A kill function which will disable the promise.
   let promise = new Promise((resolve, reject) => {
     let topic = "ipc:content-created";
@@ -27,7 +27,7 @@ function expectProcessCreated() {
 }
 
 function expectNoProcess() {
-  let os = Cc["@mozilla.org/observer-service;1"].getService(Ci.nsIObserverService);
+  let os = Services.obs;
   let topic = "ipc:content-created";
   function observer() {
     ok(false, "A process was created!");
@@ -40,9 +40,7 @@ function expectNoProcess() {
 
 function getPID(aBrowser) {
   return ContentTask.spawn(aBrowser, null, () => {
-    const appinfo = Cc["@mozilla.org/xre/app-info;1"]
-            .getService(Ci.nsIXULRuntime);
-    return appinfo.processID;
+    return Services.appinfo.processID;
   });
 }
 
@@ -103,6 +101,7 @@ async function largeAllocSuccessTests() {
     let stopExpectNoProcess = expectNoProcess();
 
     await ContentTask.spawn(aBrowser, TEST_URI, TEST_URI => {
+      // eslint-disable-next-line no-unsanitized/property
       content.document.body.innerHTML = `<iframe src='${TEST_URI}'></iframe>`;
 
       return new Promise(resolve => {
@@ -182,7 +181,6 @@ async function largeAllocSuccessTests() {
 
     await BrowserTestUtils.browserLoaded(aBrowser);
 
-    let pid3 = await getPID(aBrowser);
 
     // We should have been kicked out of the large-allocation process by the
     // load, meaning we're back in a non-fresh process
diff --git a/dom/tests/browser/helper_localStorage_e10s.js b/dom/tests/browser/helper_localStorage_e10s.js
index 307d0e84c9c9..070fef54643a 100644
--- a/dom/tests/browser/helper_localStorage_e10s.js
+++ b/dom/tests/browser/helper_localStorage_e10s.js
@@ -40,10 +40,10 @@ class KnownTabs {
  * StorageDBParent will send the set of origins when it is constructed.)
  */
 async function openTestTabInOwnProcess(helperPageUrl, name, knownTabs) {
-  let realUrl = helperPageUrl + '?' + encodeURIComponent(name);
+  let realUrl = helperPageUrl + "?" + encodeURIComponent(name);
   // Load and wait for about:blank.
   let tab = await BrowserTestUtils.openNewForegroundTab({
-    gBrowser, opening: 'about:blank', forceNewProcess: true
+    gBrowser, opening: "about:blank", forceNewProcess: true,
   });
   let pid = tab.linkedBrowser.frameLoader.tabParent.osPid;
   ok(!knownTabs.byName.has(name), "tab needs its own name: " + name);
diff --git a/dom/tests/browser/perfmetrics/browser_test_performance_metrics.js b/dom/tests/browser/perfmetrics/browser_test_performance_metrics.js
index 5c08f19d081e..b1f869a6686e 100644
--- a/dom/tests/browser/perfmetrics/browser_test_performance_metrics.js
+++ b/dom/tests/browser/perfmetrics/browser_test_performance_metrics.js
@@ -70,7 +70,6 @@ add_task(async function test() {
     let isTopLevel = false;
     let aboutMemoryFound = false;
     let parentProcessEvent = false;
-    let workerEvent = false;
     let subFrameIds = [];
     let topLevelIds = [];
     let sharedWorker = false;
@@ -108,7 +107,6 @@ add_task(async function test() {
           parentProcessEvent = true;
         }
         if (entry.isWorker) {
-          workerEvent = true;
           workerDuration += entry.duration;
         } else {
           duration += entry.duration;
diff --git a/dom/tests/browser/position.html b/dom/tests/browser/position.html
index d5b665d8e5e3..8fca4d6af982 100644
--- a/dom/tests/browser/position.html
+++ b/dom/tests/browser/position.html
@@ -5,10 +5,12 @@
     var result = null;
 
     function handlePosition(position) {
+      // eslint-disable-next-line no-unsanitized/property
       result.innerHTML = position.coords.latitude + " " + position.coords.longitude;
     }
 
     function handleError(error) {
+      // eslint-disable-next-line no-unsanitized/property
       result.innerHTML = error.message;
     }
 
diff --git a/dom/tests/browser/prevent_return_key.html b/dom/tests/browser/prevent_return_key.html
index d7dcee62ec89..d79065ce1584 100644
--- a/dom/tests/browser/prevent_return_key.html
+++ b/dom/tests/browser/prevent_return_key.html
@@ -12,6 +12,7 @@
         aEvent.preventDefault();
         return false;
       }
+      return true;
     }, {once: true});
 
     let form = document.getElementById("form");
diff --git a/dom/tests/browser/shared_worker.js b/dom/tests/browser/shared_worker.js
index f2fd14951e1e..6c9ba152499c 100644
--- a/dom/tests/browser/shared_worker.js
+++ b/dom/tests/browser/shared_worker.js
@@ -1,9 +1,7 @@
-
-onconnect = function(e) {
+let onconnect = function(e) {
   var port = e.ports[0];
 
   port.onmessage = function(e) {
-    var workerResult = "Result: " + (e.data[0] * e.data[1]);
     port.postMessage(e.data[0]);
   };
 };
diff --git a/dom/tests/browser/test-console-api.html b/dom/tests/browser/test-console-api.html
index 5bd28683334f..5cc2b980d498 100644
--- a/dom/tests/browser/test-console-api.html
+++ b/dom/tests/browser/test-console-api.html
@@ -7,6 +7,7 @@
         return a + "c";
       };
 
+      /* global foobar585956c */
       function foobar585956b(a) {
         return foobar585956c(a + "b");
       }