diff --git a/testing/marionette/harness/marionette_harness/www/dom/indexedDB/basicIDB_PBM.html b/testing/marionette/harness/marionette_harness/www/dom/indexedDB/basicIDB_PBM.html
index 90472d64d2c8..8b0b425b81bc 100644
--- a/testing/marionette/harness/marionette_harness/www/dom/indexedDB/basicIDB_PBM.html
+++ b/testing/marionette/harness/marionette_harness/www/dom/indexedDB/basicIDB_PBM.html
@@ -10,7 +10,7 @@
 					var req = indexedDB.open(name, ver);
 					req.onerror = reject;
 
-					req.onsuccess = (event) => {
+					req.onsuccess = () => {
 						resolve(req.result);
 					};
 
diff --git a/testing/marionette/harness/marionette_harness/www/html5/geolocation.js b/testing/marionette/harness/marionette_harness/www/html5/geolocation.js
index 4fb4a4747b4c..1c0132a73238 100644
--- a/testing/marionette/harness/marionette_harness/www/html5/geolocation.js
+++ b/testing/marionette/harness/marionette_harness/www/html5/geolocation.js
@@ -17,7 +17,7 @@ function success(position) {
   message.innerHTML += "<p>Altitude: " + position.coords.altitude + "</p>";
 }
 
-function error(msg) {
+function error() {
   let message = document.getElementById("status");
   message.innerHTML = "Failed to get geolocation.";
 }
diff --git a/testing/mochitest/BrowserTestUtils/BrowserTestUtils.sys.mjs b/testing/mochitest/BrowserTestUtils/BrowserTestUtils.sys.mjs
index 741882d9be7d..1793a6646fe7 100644
--- a/testing/mochitest/BrowserTestUtils/BrowserTestUtils.sys.mjs
+++ b/testing/mochitest/BrowserTestUtils/BrowserTestUtils.sys.mjs
@@ -704,7 +704,7 @@ export var BrowserTestUtils = {
    * @resolves When STATE_START reaches the tab's progress listener
    */
   browserStarted(browser, expectedURI) {
-    let testFn = function (aStateFlags, aStatus) {
+    let testFn = function (aStateFlags) {
       return (
         aStateFlags & Ci.nsIWebProgressListener.STATE_IS_NETWORK &&
         aStateFlags & Ci.nsIWebProgressListener.STATE_START
@@ -763,7 +763,7 @@ export var BrowserTestUtils = {
     } else {
       urlMatches = urlToMatch => urlToMatch != "about:blank";
     }
-    return new Promise((resolve, reject) => {
+    return new Promise(resolve => {
       tabbrowser.tabContainer.addEventListener(
         "TabOpen",
         function tabOpenListener(openEvent) {
@@ -829,15 +829,9 @@ export var BrowserTestUtils = {
    * @resolves When onLocationChange fires.
    */
   waitForLocationChange(tabbrowser, url) {
-    return new Promise((resolve, reject) => {
+    return new Promise(resolve => {
       let progressListener = {
-        onLocationChange(
-          aBrowser,
-          aWebProgress,
-          aRequest,
-          aLocationURI,
-          aFlags
-        ) {
+        onLocationChange(aBrowser, aWebProgress, aRequest, aLocationURI) {
           if (
             (url && aLocationURI.spec != url) ||
             (!url && aLocationURI.spec == "about:blank")
@@ -885,7 +879,7 @@ export var BrowserTestUtils = {
     }
 
     return new Promise((resolve, reject) => {
-      let observe = async (win, topic, data) => {
+      let observe = async (win, topic) => {
         if (topic != "domwindowopened") {
           return;
         }
@@ -1002,7 +996,7 @@ export var BrowserTestUtils = {
    */
   domWindowOpened(win, checkFn) {
     return new Promise(resolve => {
-      async function observer(subject, topic, data) {
+      async function observer(subject, topic) {
         if (topic == "domwindowopened" && (!win || subject === win)) {
           let observedWindow = subject;
           if (checkFn && !(await checkFn(observedWindow))) {
@@ -1055,7 +1049,7 @@ export var BrowserTestUtils = {
    */
   domWindowClosed(win) {
     return new Promise(resolve => {
-      function observer(subject, topic, data) {
+      function observer(subject, topic) {
         if (topic == "domwindowclosed" && (!win || subject === win)) {
           Services.ww.unregisterNotification(observer);
           resolve(subject);
@@ -1167,7 +1161,7 @@ export var BrowserTestUtils = {
           win.gBrowser._insertBrowser(win.gBrowser.getTabForBrowser(browser));
         });
 
-        let observer = (subject, topic, data) => {
+        let observer = subject => {
           if (browserSet.has(subject)) {
             browserSet.delete(subject);
           }
@@ -1202,7 +1196,7 @@ export var BrowserTestUtils = {
     return new Promise(resolve => {
       let browser = tab.linkedBrowser;
       let flushTopic = "sessionstore-browser-shutdown-flush";
-      let observer = (subject, topic, data) => {
+      let observer = subject => {
         if (subject === browser) {
           Services.obs.removeObserver(observer, flushTopic);
           // Wait for the next event tick to make sure other listeners are
@@ -1615,7 +1609,7 @@ export var BrowserTestUtils = {
     }
   },
 
-  observe(subject, topic, data) {
+  observe(subject, topic) {
     switch (topic) {
       case "test-complete":
         this._cleanupContentEventListeners();
@@ -2054,7 +2048,7 @@ export var BrowserTestUtils = {
     let expectedPromises = [];
 
     let crashCleanupPromise = new Promise((resolve, reject) => {
-      let observer = (subject, topic, data) => {
+      let observer = (subject, topic) => {
         if (topic != "ipc:content-shutdown") {
           reject("Received incorrect observer topic: " + topic);
           return;
@@ -2129,7 +2123,7 @@ export var BrowserTestUtils = {
 
     if (shouldShowTabCrashPage) {
       expectedPromises.push(
-        new Promise((resolve, reject) => {
+        new Promise(resolve => {
           browser.addEventListener(
             "AboutTabCrashedReady",
             function onCrash() {
@@ -2187,7 +2181,7 @@ export var BrowserTestUtils = {
     });
 
     let sawNormalCrash = false;
-    let observer = (subject, topic, data) => {
+    let observer = () => {
       sawNormalCrash = true;
     };
 
@@ -2232,7 +2226,7 @@ export var BrowserTestUtils = {
   waitForAttribute(attr, element, value) {
     let MutationObserver = element.ownerGlobal.MutationObserver;
     return new Promise(resolve => {
-      let mut = new MutationObserver(mutations => {
+      let mut = new MutationObserver(() => {
         if (
           (!value && element.hasAttribute(attr)) ||
           (value && element.getAttribute(attr) === value)
@@ -2263,7 +2257,7 @@ export var BrowserTestUtils = {
     let MutationObserver = element.ownerGlobal.MutationObserver;
     return new Promise(resolve => {
       dump("Waiting for removal\n");
-      let mut = new MutationObserver(mutations => {
+      let mut = new MutationObserver(() => {
         if (!element.hasAttribute(attr)) {
           resolve();
           mut.disconnect();
diff --git a/testing/mochitest/BrowserTestUtils/content/content-about-page-utils.js b/testing/mochitest/BrowserTestUtils/content/content-about-page-utils.js
index e20100db4bfe..759e283bcb59 100644
--- a/testing/mochitest/BrowserTestUtils/content/content-about-page-utils.js
+++ b/testing/mochitest/BrowserTestUtils/content/content-about-page-utils.js
@@ -14,7 +14,7 @@ function AboutPage(aboutHost, chromeURL, uriFlags) {
 
 AboutPage.prototype = {
   QueryInterface: ChromeUtils.generateQI(["nsIAboutModule"]),
-  getURIFlags(aURI) {
+  getURIFlags() {
     // eslint-disable-line no-unused-vars
     return this.uriFlags;
   },
diff --git a/testing/mochitest/api.js b/testing/mochitest/api.js
index bd578f03b873..fa4422f2900a 100644
--- a/testing/mochitest/api.js
+++ b/testing/mochitest/api.js
@@ -26,7 +26,7 @@ const windowTracker = {
     Services.obs.addObserver(this, "chrome-document-global-created");
   },
 
-  async observe(window, topic, data) {
+  async observe(window, topic) {
     if (topic === "chrome-document-global-created") {
       await new Promise(resolve =>
         window.addEventListener("DOMContentLoaded", resolve, { once: true })
diff --git a/testing/mochitest/baselinecoverage/browser_chrome/browser_baselinecoverage_browser-chrome.js b/testing/mochitest/baselinecoverage/browser_chrome/browser_baselinecoverage_browser-chrome.js
index 4015bd6edf57..3095934ffdaa 100644
--- a/testing/mochitest/baselinecoverage/browser_chrome/browser_baselinecoverage_browser-chrome.js
+++ b/testing/mochitest/baselinecoverage/browser_chrome/browser_baselinecoverage_browser-chrome.js
@@ -13,7 +13,7 @@ add_task(async function () {
       gBrowser,
       url: "about:blank",
     },
-    async function (browser) {
+    async function () {
       ok(true, "Collecting baseline coverage for browser-chrome tests.");
       await new Promise(c => setTimeout(c, 30 * 1000));
     }
diff --git a/testing/mochitest/browser-test.js b/testing/mochitest/browser-test.js
index 98b905560556..1f9798bb368a 100644
--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -45,7 +45,7 @@ var TabDestroyObserver = {
     Services.obs.removeObserver(this, "message-manager-disconnect");
   },
 
-  observe(subject, topic, data) {
+  observe(subject, topic) {
     if (topic == "message-manager-close") {
       this.outstanding.add(subject);
     } else if (topic == "message-manager-disconnect") {
@@ -516,7 +516,7 @@ Tester.prototype = {
     this.SimpleTest.waitForFocus(aCallback);
   },
 
-  finish: function Tester_finish(aSkipSummary) {
+  finish: function Tester_finish() {
     var passCount = this.tests.reduce((a, f) => a + f.passCount, 0);
     var failCount = this.tests.reduce((a, f) => a + f.failCount, 0);
     var todoCount = this.tests.reduce((a, f) => a + f.todoCount, 0);
@@ -564,7 +564,7 @@ Tester.prototype = {
     this.repeat = 0;
   },
 
-  observe: function Tester_observe(aSubject, aTopic, aData) {
+  observe: function Tester_observe(aSubject, aTopic) {
     if (!aTopic) {
       this.onConsoleMessage(aSubject);
     }
diff --git a/testing/mochitest/server.js b/testing/mochitest/server.js
index 740404d94ea1..934f0b281cb7 100644
--- a/testing/mochitest/server.js
+++ b/testing/mochitest/server.js
@@ -215,7 +215,7 @@ function createMochitestServer(serverBasePath) {
       });
       return file;
     },
-    QueryInterface(aIID) {
+    QueryInterface() {
       return this;
     },
   };
diff --git a/testing/mochitest/tests/Harness_sanity/SpecialPowersLoadChromeScript.js b/testing/mochitest/tests/Harness_sanity/SpecialPowersLoadChromeScript.js
index 9591f3bca483..92a84e1ffa0c 100644
--- a/testing/mochitest/tests/Harness_sanity/SpecialPowersLoadChromeScript.js
+++ b/testing/mochitest/tests/Harness_sanity/SpecialPowersLoadChromeScript.js
@@ -6,7 +6,7 @@ addMessageListener("foo", function (message) {
   sendAsyncMessage("bar", message);
 });
 
-addMessageListener("valid-assert", function (message) {
+addMessageListener("valid-assert", function () {
   assert.ok(true, "valid assertion");
   assert.equal(1, 1, "another valid assertion");
   sendAsyncMessage("valid-assert-done");
diff --git a/testing/mochitest/tests/Harness_sanity/test_SpecialPowersExtension.html b/testing/mochitest/tests/Harness_sanity/test_SpecialPowersExtension.html
index 34b75933a037..3d374735a6a6 100644
--- a/testing/mochitest/tests/Harness_sanity/test_SpecialPowersExtension.html
+++ b/testing/mochitest/tests/Harness_sanity/test_SpecialPowersExtension.html
@@ -14,11 +14,11 @@
 <script class="testbody" type="text/javascript">
 
 var eventCount = 0;
-function testEventListener(e) {
+function testEventListener() {
   ++eventCount;
 }
 
-function testEventListener2(e) {
+function testEventListener2() {
   ++eventCount;
 }
 
diff --git a/testing/mochitest/tests/Harness_sanity/test_SpecialPowersLoadChromeScript.html b/testing/mochitest/tests/Harness_sanity/test_SpecialPowersLoadChromeScript.html
index 7242bd19f591..3c9bbf391313 100644
--- a/testing/mochitest/tests/Harness_sanity/test_SpecialPowersLoadChromeScript.html
+++ b/testing/mochitest/tests/Harness_sanity/test_SpecialPowersLoadChromeScript.html
@@ -37,7 +37,7 @@ function endOfFirstTest() {
   // that is used to run the chrome script.
   script2 = SpecialPowers.loadChromeScript(_ => {
     /* eslint-env mozilla/chrome-script */
-    addMessageListener("valid-assert", function (message) {
+    addMessageListener("valid-assert", function () {
       assert.equal(typeof XMLHttpRequest, "function", "XMLHttpRequest is defined");
       assert.equal(typeof CSS, "undefined", "CSS is not defined");
       sendAsyncMessage("valid-assert-done");
diff --git a/testing/mochitest/tests/Harness_sanity/test_SpecialPowersLoadChromeScript_function.html b/testing/mochitest/tests/Harness_sanity/test_SpecialPowersLoadChromeScript_function.html
index af31d7b25a95..4de074800f49 100644
--- a/testing/mochitest/tests/Harness_sanity/test_SpecialPowersLoadChromeScript_function.html
+++ b/testing/mochitest/tests/Harness_sanity/test_SpecialPowersLoadChromeScript_function.html
@@ -22,7 +22,7 @@ var script = SpecialPowers.loadChromeScript(function loadChromeScriptTest() {
     sendAsyncMessage("bar", message);
   });
 
-  addMessageListener("valid-assert", function (message) {
+  addMessageListener("valid-assert", function () {
     assert.ok(true, "valid assertion");
     assert.equal(1, 1, "another valid assertion");
     sendAsyncMessage("valid-assert-done");
diff --git a/testing/mochitest/tests/Harness_sanity/test_createFiles.html b/testing/mochitest/tests/Harness_sanity/test_createFiles.html
index 0e0637a23091..7a06a2ad1155 100644
--- a/testing/mochitest/tests/Harness_sanity/test_createFiles.html
+++ b/testing/mochitest/tests/Harness_sanity/test_createFiles.html
@@ -23,7 +23,7 @@
           is(f.type, fileType, "File should have the specified type");
           test2();
         },
-        function (msg) { ok(false, "Should be able to create a file without an error"); test2(); }
+        function () { ok(false, "Should be able to create a file without an error"); test2(); }
       );
     }
 
@@ -36,7 +36,7 @@
 
       SpecialPowers.createFiles([{name: "/\/\/\/\/\/\/\/\/\/\/\invalidname",}],
         function () { test3Check(false); },
-        function (msg) { test3Check(true); }
+        function () { test3Check(true); }
       );
     }
 
@@ -75,7 +75,7 @@
           ok(f.name, "test4 test file should have a name");
           SimpleTest.finish();
         },
-        function (msg) {
+        function () {
           ok(false, "Should be able to create a file without a name without an error");
           SimpleTest.finish();
         }
diff --git a/testing/mochitest/tests/Harness_sanity/test_sanityEventUtils.html b/testing/mochitest/tests/Harness_sanity/test_sanityEventUtils.html
index b24251b227e5..4a9abb230598 100644
--- a/testing/mochitest/tests/Harness_sanity/test_sanityEventUtils.html
+++ b/testing/mochitest/tests/Harness_sanity/test_sanityEventUtils.html
@@ -247,7 +247,7 @@ function starttest() {
 
       /* test synthesizeMouse* */
       //focus trick enables us to run this in iframes
-      $("radioTarget1").addEventListener('focus', function (aEvent) {
+      $("radioTarget1").addEventListener('focus', function () {
         synthesizeMouse($("radioTarget1"), 1, 1, {});
         is($("radioTarget1").checked, true, "synthesizeMouse should work")
         $("radioTarget1").checked = false;
@@ -260,7 +260,7 @@ function starttest() {
       $("radioTarget1").focus();
 
       //focus trick enables us to run this in iframes
-      $("textBoxA").addEventListener("focus", function (aEvent) {
+      $("textBoxA").addEventListener("focus", function () {
         check = false;
         $("textBoxA").addEventListener("click", function() { check = true; }, { once: true });
         synthesizeMouseAtCenter($("textBoxA"), {});
diff --git a/testing/mochitest/tests/SimpleTest/EventUtils.js b/testing/mochitest/tests/SimpleTest/EventUtils.js
index 739c7052ea1f..43d5aa02c331 100644
--- a/testing/mochitest/tests/SimpleTest/EventUtils.js
+++ b/testing/mochitest/tests/SimpleTest/EventUtils.js
@@ -1405,13 +1405,10 @@ function synthesizeAndWaitNativeMouseMove(
   );
 
   let eventRegisteredPromise = new Promise(resolve => {
-    mm.addMessageListener(
-      "Test:MouseMoveRegistered",
-      function processed(message) {
-        mm.removeMessageListener("Test:MouseMoveRegistered", processed);
-        resolve();
-      }
-    );
+    mm.addMessageListener("Test:MouseMoveRegistered", function processed() {
+      mm.removeMessageListener("Test:MouseMoveRegistered", processed);
+      resolve();
+    });
   });
   let eventReceivedPromise = ContentTask.spawn(
     browser,
@@ -1579,7 +1576,7 @@ function synthesizeAndWaitKey(
   );
 
   let keyRegisteredPromise = new Promise(resolve => {
-    mm.addMessageListener("Test:KeyRegistered", function processed(message) {
+    mm.addMessageListener("Test:KeyRegistered", function processed() {
       mm.removeMessageListener("Test:KeyRegistered", processed);
       resolve();
     });
@@ -3884,7 +3881,7 @@ class EventCounter {
     // SpecialPowers is picky and needs to be passed an explicit reference to
     // the function to be called. To avoid having to bind "this", we therefore
     // define the method this way, via a property.
-    this.handleEvent = aEvent => {
+    this.handleEvent = () => {
       this.eventCount++;
     };
 
diff --git a/testing/mochitest/tests/SimpleTest/LogController.js b/testing/mochitest/tests/SimpleTest/LogController.js
index 29580022f80e..d0888d603204 100644
--- a/testing/mochitest/tests/SimpleTest/LogController.js
+++ b/testing/mochitest/tests/SimpleTest/LogController.js
@@ -48,7 +48,7 @@ LogController.extend = function (args, skip) {
 };
 
 /* logs message with given level. Currently used locally by log() and error() */
-LogController.logWithLevel = function (level, message /*, ...*/) {
+LogController.logWithLevel = function (level /*, ...*/) {
   var msg = LogController.createLogMessage(
     level,
     LogController.extend(arguments, 1)
diff --git a/testing/mochitest/tests/SimpleTest/SimpleTest.js b/testing/mochitest/tests/SimpleTest/SimpleTest.js
index a237c25103d2..16f5c3ae8a13 100644
--- a/testing/mochitest/tests/SimpleTest/SimpleTest.js
+++ b/testing/mochitest/tests/SimpleTest/SimpleTest.js
@@ -384,7 +384,7 @@ function usesFailurePatterns() {
  * @return {boolean} Whether a matched failure pattern is found.
  */
 function recordIfMatchesFailurePattern(name, diag) {
-  let index = SimpleTest.expected.findIndex(([pat, count]) => {
+  let index = SimpleTest.expected.findIndex(([pat]) => {
     return (
       pat == null ||
       (typeof name == "string" && name.includes(pat)) ||
diff --git a/testing/mochitest/tests/SimpleTest/TestRunner.js b/testing/mochitest/tests/SimpleTest/TestRunner.js
index 5f305a176bef..2e48969d8d86 100644
--- a/testing/mochitest/tests/SimpleTest/TestRunner.js
+++ b/testing/mochitest/tests/SimpleTest/TestRunner.js
@@ -63,7 +63,7 @@ function extend(obj, /* optional */ skip) {
   return ret;
 }
 
-function flattenArguments(lst /* ...*/) {
+function flattenArguments(/* ...*/) {
   var res = [];
   var args = extend(arguments);
   while (args.length) {
diff --git a/testing/mochitest/tests/SimpleTest/paint_listener.js b/testing/mochitest/tests/SimpleTest/paint_listener.js
index 2fc6ab425a50..89740454f286 100644
--- a/testing/mochitest/tests/SimpleTest/paint_listener.js
+++ b/testing/mochitest/tests/SimpleTest/paint_listener.js
@@ -100,7 +100,7 @@
 
   window.promiseAllPaintsDone = function (subdoc = null, flush = false) {
     var flushmode = flush ? FlushModes.FLUSH : FlushModes.NOFLUSH;
-    return new Promise(function (resolve, reject) {
+    return new Promise(function (resolve) {
       // The callback is given the components of the rect, but resolve() can
       // only be given one arg, so we turn it back into an array.
       waitForPaints((l, r, t, b) => resolve([l, r, t, b]), subdoc, flushmode);
diff --git a/testing/mochitest/tests/SimpleTest/setup.js b/testing/mochitest/tests/SimpleTest/setup.js
index 05ba8066a1a6..2a8509bc0af2 100644
--- a/testing/mochitest/tests/SimpleTest/setup.js
+++ b/testing/mochitest/tests/SimpleTest/setup.js
@@ -247,7 +247,7 @@ TestRunner.logger.addListener(
 var gTestList = [];
 var RunSet = {};
 
-RunSet.runall = function (e) {
+RunSet.runall = function () {
   // Filter tests to include|exclude tests based on data in params.filter.
   // This allows for including or excluding tests from the gTestList
   // TODO Only used by ipc tests, remove once those are implemented sanely
@@ -265,7 +265,7 @@ RunSet.runall = function (e) {
   }
 };
 
-RunSet.runtests = function (e) {
+RunSet.runtests = function () {
   // Which tests we're going to run
   var my_tests = gTestList;
 
diff --git a/testing/modules/MockRegistry.sys.mjs b/testing/modules/MockRegistry.sys.mjs
index e9351e576da8..b64d1ce2a859 100644
--- a/testing/modules/MockRegistry.sys.mjs
+++ b/testing/modules/MockRegistry.sys.mjs
@@ -34,7 +34,7 @@ export class MockRegistry {
       QueryInterface: ChromeUtils.generateQI(["nsIWindowsRegKey"]),
 
       // --- Overridden nsIWindowsRegKey interface functions ---
-      open(root, path, mode) {
+      open(root, path) {
         let rootKey = registry.getRoot(root);
         if (!rootKey.has(path)) {
           rootKey.set(path, new Map());
@@ -60,7 +60,7 @@ export class MockRegistry {
         return this.values.has(name);
       },
 
-      getValueType(name) {
+      getValueType() {
         return Ci.nsIWindowsRegKey.TYPE_STRING;
       },
 
diff --git a/testing/modules/TestUtils.sys.mjs b/testing/modules/TestUtils.sys.mjs
index 51030f5fbccf..cbf6b2d439cd 100644
--- a/testing/modules/TestUtils.sys.mjs
+++ b/testing/modules/TestUtils.sys.mjs
@@ -207,33 +207,30 @@ export var TestUtils = {
    */
   waitForPrefChange(prefName, checkFn) {
     return new Promise((resolve, reject) => {
-      Services.prefs.addObserver(
-        prefName,
-        function observer(subject, topic, data) {
-          try {
-            let prefValue = null;
-            switch (Services.prefs.getPrefType(prefName)) {
-              case Services.prefs.PREF_STRING:
-                prefValue = Services.prefs.getStringPref(prefName);
-                break;
-              case Services.prefs.PREF_INT:
-                prefValue = Services.prefs.getIntPref(prefName);
-                break;
-              case Services.prefs.PREF_BOOL:
-                prefValue = Services.prefs.getBoolPref(prefName);
-                break;
-            }
-            if (checkFn && !checkFn(prefValue)) {
-              return;
-            }
-            Services.prefs.removeObserver(prefName, observer);
-            resolve(prefValue);
-          } catch (ex) {
-            Services.prefs.removeObserver(prefName, observer);
-            reject(ex);
+      Services.prefs.addObserver(prefName, function observer() {
+        try {
+          let prefValue = null;
+          switch (Services.prefs.getPrefType(prefName)) {
+            case Services.prefs.PREF_STRING:
+              prefValue = Services.prefs.getStringPref(prefName);
+              break;
+            case Services.prefs.PREF_INT:
+              prefValue = Services.prefs.getIntPref(prefName);
+              break;
+            case Services.prefs.PREF_BOOL:
+              prefValue = Services.prefs.getBoolPref(prefName);
+              break;
           }
+          if (checkFn && !checkFn(prefValue)) {
+            return;
+          }
+          Services.prefs.removeObserver(prefName, observer);
+          resolve(prefValue);
+        } catch (ex) {
+          Services.prefs.removeObserver(prefName, observer);
+          reject(ex);
         }
-      );
+      });
     });
   },
 
diff --git a/testing/modules/XPCShellContentUtils.sys.mjs b/testing/modules/XPCShellContentUtils.sys.mjs
index 21ac5dbae48d..118c3fc5c7c9 100644
--- a/testing/modules/XPCShellContentUtils.sys.mjs
+++ b/testing/modules/XPCShellContentUtils.sys.mjs
@@ -68,7 +68,7 @@ function promiseBrowserLoaded(browser, url, redirectUrl) {
         "nsIWebProgressListener",
       ]),
 
-      onStateChange(webProgress, request, stateFlags, statusCode) {
+      onStateChange(webProgress, request, stateFlags) {
         request.QueryInterface(Ci.nsIChannel);
 
         let requestURI =
@@ -233,7 +233,7 @@ class ContentPage {
     this.browser.messageManager.loadFrameScript(frameScript, false, true);
   }
 
-  didChangeBrowserRemoteness(event) {
+  didChangeBrowserRemoteness() {
     // XXX: Tests can load their own additional frame scripts, so we may need to
     // track all scripts that have been loaded, and reload them here?
     this.loadFrameScript(frameScript);
diff --git a/testing/mozbase/mozlog/mozlog/formatters/html/main.js b/testing/mozbase/mozlog/mozlog/formatters/html/main.js
index 61403f611b12..62ccf191aaad 100644
--- a/testing/mozbase/mozlog/mozlog/formatters/html/main.js
+++ b/testing/mozbase/mozlog/mozlog/formatters/html/main.js
@@ -52,7 +52,7 @@ addEventListener("DOMContentLoaded", function () {
   });
 
   find_all(".sortable").forEach(function (elem) {
-    elem.addEventListener("click", function (event) {
+    elem.addEventListener("click", function () {
       toggle_sort_states(elem);
       var colIndex = toArray(elem.parentNode.childNodes).indexOf(elem);
       var key = elem.classList.contains("numeric") ? key_num : key_alpha;
diff --git a/testing/performance/perftest_android_startup.js b/testing/performance/perftest_android_startup.js
index 4dfcd32af068..55f17dca6edd 100644
--- a/testing/performance/perftest_android_startup.js
+++ b/testing/performance/perftest_android_startup.js
@@ -4,7 +4,7 @@
 /* eslint-env node */
 "use strict";
 
-async function test(context, commands) {}
+async function test() {}
 
 module.exports = {
   test,
diff --git a/testing/specialpowers/content/SpecialPowersChild.sys.mjs b/testing/specialpowers/content/SpecialPowersChild.sys.mjs
index 65c806fe8608..790374269500 100644
--- a/testing/specialpowers/content/SpecialPowersChild.sys.mjs
+++ b/testing/specialpowers/content/SpecialPowersChild.sys.mjs
@@ -69,7 +69,7 @@ function SPConsoleListener(callback, contentWindow) {
 SPConsoleListener.prototype = {
   // Overload the observe method for both nsIConsoleListener and nsIObserver.
   // The topic will be null for nsIConsoleListener.
-  observe(msg, topic) {
+  observe(msg) {
     let m = {
       message: msg.message,
       errorMessage: null,
@@ -162,7 +162,7 @@ export class SpecialPowersChild extends JSWindowActorChild {
     );
   }
 
-  observe(aSubject, aTopic, aData) {
+  observe() {
     // Ignore the "{chrome/content}-document-global-created" event. It
     // is only observed to force creation of the actor.
   }
@@ -1089,7 +1089,7 @@ export class SpecialPowersChild extends JSWindowActorChild {
     }
     return val;
   }
-  _getPref(prefName, prefType, { defaultValue }) {
+  _getPref(prefName, prefType) {
     switch (prefType) {
       case "BOOL":
         return Services.prefs.getBoolPref(prefName);
@@ -1132,7 +1132,7 @@ export class SpecialPowersChild extends JSWindowActorChild {
   removeAutoCompletePopupEventListener(window, eventname, listener) {
     this._getAutoCompletePopup(window).removeEventListener(eventname, listener);
   }
-  getFormFillController(window) {
+  getFormFillController() {
     return Cc["@mozilla.org/satchel/form-fill-controller;1"].getService(
       Ci.nsIFormFillController
     );
@@ -1927,11 +1927,11 @@ export class SpecialPowersChild extends JSWindowActorChild {
     return this.sendQuery("SPRemoveServiceWorkerDataForExampleDomain", {});
   }
 
-  cleanUpSTSData(origin, flags) {
+  cleanUpSTSData(origin) {
     return this.sendQuery("SPCleanUpSTSData", { origin });
   }
 
-  async requestDumpCoverageCounters(cb) {
+  async requestDumpCoverageCounters() {
     // We want to avoid a roundtrip between child and parent.
     if (!lazy.PerTestCoverageUtils.enabled) {
       return;
@@ -1940,7 +1940,7 @@ export class SpecialPowersChild extends JSWindowActorChild {
     await this.sendQuery("SPRequestDumpCoverageCounters", {});
   }
 
-  async requestResetCoverageCounters(cb) {
+  async requestResetCoverageCounters() {
     // We want to avoid a roundtrip between child and parent.
     if (!lazy.PerTestCoverageUtils.enabled) {
       return;
@@ -2275,7 +2275,7 @@ SpecialPowersChild.prototype._proxiedObservers = {
     );
   },
 
-  "specialpowers-service-worker-shutdown": function (aMessage) {
+  "specialpowers-service-worker-shutdown": function () {
     Services.obs.notifyObservers(null, "specialpowers-service-worker-shutdown");
   },
 
diff --git a/testing/specialpowers/content/SpecialPowersParent.sys.mjs b/testing/specialpowers/content/SpecialPowersParent.sys.mjs
index 3ed660d71ca3..e0ce35bb6765 100644
--- a/testing/specialpowers/content/SpecialPowersParent.sys.mjs
+++ b/testing/specialpowers/content/SpecialPowersParent.sys.mjs
@@ -247,7 +247,7 @@ export class SpecialPowersParent extends JSWindowActorParent {
     swm.removeListener(this._serviceWorkerListener);
   }
 
-  observe(aSubject, aTopic, aData) {
+  observe(aSubject, aTopic) {
     function addDumpIDToMessage(propertyName) {
       try {
         var id = aSubject.getPropertyAsAString(propertyName);
diff --git a/testing/specialpowers/content/WrapPrivileged.sys.mjs b/testing/specialpowers/content/WrapPrivileged.sys.mjs
index 5fe67a94e2e9..1db8a7d9f7ed 100644
--- a/testing/specialpowers/content/WrapPrivileged.sys.mjs
+++ b/testing/specialpowers/content/WrapPrivileged.sys.mjs
@@ -257,7 +257,7 @@ let SpecialPowersHandler = {
     return Reflect.has(this.wrapped.get(target).obj, prop);
   },
 
-  get(target, prop, receiver) {
+  get(target, prop) {
     let global = Cu.getGlobalForObject(this);
     return wrapExceptions(global, () => {
       let obj = waiveXraysIfAppropriate(this.wrapped.get(target).obj, prop);
@@ -266,7 +266,7 @@ let SpecialPowersHandler = {
     });
   },
 
-  set(target, prop, val, receiver) {
+  set(target, prop, val) {
     return wrapExceptions(Cu.getGlobalForObject(this), () => {
       let obj = waiveXraysIfAppropriate(this.wrapped.get(target).obj, prop);
       return Reflect.set(obj, prop, unwrapIfWrapped(val));
@@ -279,7 +279,7 @@ let SpecialPowersHandler = {
     });
   },
 
-  defineProperty(target, prop, descriptor) {
+  defineProperty() {
     throw new Error(
       "Can't call defineProperty on SpecialPowers wrapped object"
     );
@@ -332,7 +332,7 @@ let SpecialPowersHandler = {
     return Cu.cloneInto(props, Cu.getGlobalForObject(this));
   },
 
-  preventExtensions(target) {
+  preventExtensions() {
     throw new Error(
       "Can't call preventExtensions on SpecialPowers wrapped object"
     );
diff --git a/testing/talos/talos/bootstrap.js b/testing/talos/talos/bootstrap.js
index 7acad7ea905e..22749a28fb5e 100644
--- a/testing/talos/talos/bootstrap.js
+++ b/testing/talos/talos/bootstrap.js
@@ -27,7 +27,7 @@ const windowTracker = {
     Services.ww.registerNotification(this);
   },
 
-  async observe(window, topic, data) {
+  async observe(window, topic) {
     if (topic === "domwindowopened") {
       await new Promise(resolve =>
         window.addEventListener("DOMWindowCreated", resolve, { once: true })
@@ -50,13 +50,13 @@ function readSync(uri) {
   return new TextDecoder().decode(buffer);
 }
 
-function startup(data, reason) {
+function startup(data) {
   Services.scriptloader.loadSubScript(
     data.resourceURI.resolve("content/initialize_browser.js")
   );
   windowTracker.init();
 }
 
-function shutdown(data, reason) {}
-function install(data, reason) {}
-function uninstall(data, reason) {}
+function shutdown() {}
+function install() {}
+function uninstall() {}
diff --git a/testing/talos/talos/pageloader/chrome/a11y.js b/testing/talos/talos/pageloader/chrome/a11y.js
index 3541076ee744..981a1700ac69 100644
--- a/testing/talos/talos/pageloader/chrome/a11y.js
+++ b/testing/talos/talos/pageloader/chrome/a11y.js
@@ -55,7 +55,7 @@
     ensureAccessibleTreeForNode(node);
   }
 
-  addEventListener("DOMContentLoaded", e => {
+  addEventListener("DOMContentLoaded", () => {
     Cu.exportFunction(initAccessibility, content, {
       defineAs: "initAccessibility",
     });
diff --git a/testing/talos/talos/pageloader/chrome/lh_hero.js b/testing/talos/talos/pageloader/chrome/lh_hero.js
index 6705b6d53c7b..55f28e233383 100644
--- a/testing/talos/talos/pageloader/chrome/lh_hero.js
+++ b/testing/talos/talos/pageloader/chrome/lh_hero.js
@@ -8,8 +8,8 @@ function _contentHeroHandler(isload) {
   var obs = null;
   var el = content.window.document.querySelector("[elementtiming]");
   if (el) {
-    function callback(entries, observer) {
-      entries.forEach(entry => {
+    function callback(entries) {
+      entries.forEach(() => {
         sendAsyncMessage("PageLoader:LoadEvent", {
           time: content.window.performance.now(),
           name: "tphero",
diff --git a/testing/talos/talos/pageloader/chrome/lh_moz.js b/testing/talos/talos/pageloader/chrome/lh_moz.js
index 46d4a74a7c23..8879d172e254 100644
--- a/testing/talos/talos/pageloader/chrome/lh_moz.js
+++ b/testing/talos/talos/pageloader/chrome/lh_moz.js
@@ -9,7 +9,7 @@ function _contentPaintHandler() {
   if (utils.isMozAfterPaintPending) {
     addEventListener(
       "MozAfterPaint",
-      function afterpaint(e) {
+      function afterpaint() {
         removeEventListener("MozAfterPaint", afterpaint, true);
         sendAsyncMessage("PageLoader:LoadEvent", {});
       },
diff --git a/testing/talos/talos/pageloader/chrome/pageloader.js b/testing/talos/talos/pageloader/chrome/pageloader.js
index 839901fb8e60..6b1f89f71dc3 100644
--- a/testing/talos/talos/pageloader/chrome/pageloader.js
+++ b/testing/talos/talos/pageloader/chrome/pageloader.js
@@ -241,7 +241,7 @@ async function plInit() {
     // pages should be able to load in the same mode as the initial page - due
     // to this reinitialization on the switch.
     let tab = gBrowser.selectedTab;
-    tab.addEventListener("TabRemotenessChange", function (evt) {
+    tab.addEventListener("TabRemotenessChange", function () {
       loadFrameScripts(tab.linkedBrowser);
     });
     loadFrameScripts(tab.linkedBrowser);
@@ -300,7 +300,7 @@ function plLoadPage() {
   }
 
   let tab = gBrowser.selectedTab;
-  tab.addEventListener("TabRemotenessChange", evt => {
+  tab.addEventListener("TabRemotenessChange", () => {
     addMsgListeners(tab.linkedBrowser);
   });
   addMsgListeners(tab.linkedBrowser);
@@ -581,13 +581,10 @@ function waitForPDFPaint() {
 function forceContentGC() {
   return new Promise(resolve => {
     let mm = browserWindow.gBrowser.selectedBrowser.messageManager;
-    mm.addMessageListener(
-      "Talos:ForceGC:OK",
-      function onTalosContentForceGC(msg) {
-        mm.removeMessageListener("Talos:ForceGC:OK", onTalosContentForceGC);
-        resolve();
-      }
-    );
+    mm.addMessageListener("Talos:ForceGC:OK", function onTalosContentForceGC() {
+      mm.removeMessageListener("Talos:ForceGC:OK", onTalosContentForceGC);
+      resolve();
+    });
     mm.sendAsyncMessage("Talos:ForceGC");
   });
 }
diff --git a/testing/talos/talos/pageloader/chrome/tscroll.js b/testing/talos/talos/pageloader/chrome/tscroll.js
index 7f64b8c4744d..e354a474fbc4 100644
--- a/testing/talos/talos/pageloader/chrome/tscroll.js
+++ b/testing/talos/talos/pageloader/chrome/tscroll.js
@@ -267,7 +267,7 @@ function testScroll(target, stepSize, opt_reportFunc, opt_numSteps) {
       TalosPowersParent.exec("stopFrameTimeRecording", handle, cb, win);
     }
 
-    return new Promise(function (resolve, reject) {
+    return new Promise(function (resolve) {
       setSmooth();
 
       var handle = -1;
diff --git a/testing/talos/talos/startup_test/sessionrestore/addon/api.js b/testing/talos/talos/startup_test/sessionrestore/addon/api.js
index f621894623a1..87f0d46aee8e 100644
--- a/testing/talos/talos/startup_test/sessionrestore/addon/api.js
+++ b/testing/talos/talos/startup_test/sessionrestore/addon/api.js
@@ -28,7 +28,7 @@ this.sessionrestore = class extends ExtensionAPI {
     this.run();
   }
 
-  observe(subject, topic, data) {
+  observe(subject, topic) {
     if (topic == "browser-idle-startup-tasks-finished") {
       this.promiseIdleFinished.resolve();
     }
diff --git a/testing/talos/talos/startup_test/startup_about_home_paint/addon/api.js b/testing/talos/talos/startup_test/startup_about_home_paint/addon/api.js
index 14887aa11230..9367dec45ea6 100644
--- a/testing/talos/talos/startup_test/startup_about_home_paint/addon/api.js
+++ b/testing/talos/talos/startup_test/startup_about_home_paint/addon/api.js
@@ -27,7 +27,7 @@ this.startup_about_home_paint = class extends ExtensionAPI {
     });
   }
 
-  observe(subject, topic, data) {
+  observe(subject, topic) {
     if (topic == "browser-idle-startup-tasks-finished") {
       this.checkForTelemetry();
     }
diff --git a/testing/talos/talos/talos-powers/api.js b/testing/talos/talos/talos-powers/api.js
index cb0ddaccefe4..c6b761197650 100644
--- a/testing/talos/talos/talos-powers/api.js
+++ b/testing/talos/talos/talos-powers/api.js
@@ -371,7 +371,7 @@ TalosPowersService.prototype = {
     },
   */
   ParentExecServices: {
-    ping(arg, callback, win) {
+    ping(arg, callback) {
       callback();
     },
 
@@ -387,15 +387,15 @@ TalosPowersService.prototype = {
       callback(rv);
     },
 
-    requestDumpCoverageCounters(arg, callback, win) {
+    requestDumpCoverageCounters(arg, callback) {
       PerTestCoverageUtils.afterTest().then(callback);
     },
 
-    requestResetCoverageCounters(arg, callback, win) {
+    requestResetCoverageCounters(arg, callback) {
       PerTestCoverageUtils.beforeTest().then(callback);
     },
 
-    dumpAboutSupport(arg, callback, win) {
+    dumpAboutSupport(arg, callback) {
       const { Troubleshoot } = ChromeUtils.importESModule(
         "resource://gre/modules/Troubleshoot.sys.mjs"
       );
diff --git a/testing/talos/talos/talos-powers/chrome/talos-powers-content.js b/testing/talos/talos/talos-powers/chrome/talos-powers-content.js
index 4b7276e7a59a..79e1742d1a2a 100644
--- a/testing/talos/talos/talos-powers/chrome/talos-powers-content.js
+++ b/testing/talos/talos/talos-powers/chrome/talos-powers-content.js
@@ -35,7 +35,7 @@ addMessageListener("TalosContentProfiler:Response", msg => {
 
 addEventListener(
   "TalosPowersContentForceCCAndGC",
-  e => {
+  () => {
     Cu.forceGC();
     Cu.forceCC();
     Cu.forceShrinkingGC();
@@ -46,7 +46,7 @@ addEventListener(
 
 addEventListener(
   "TalosPowersContentFocus",
-  e => {
+  () => {
     if (
       content.location.protocol != "file:" &&
       content.location.hostname != "localhost" &&
@@ -72,7 +72,7 @@ addEventListener(
 
 addEventListener(
   "TalosPowersContentGetStartupInfo",
-  e => {
+  () => {
     sendAsyncMessage("TalosPowersContent:GetStartupInfo");
     addMessageListener(
       "TalosPowersContent:GetStartupInfo:Result",
@@ -103,7 +103,7 @@ addEventListener(
 
 addEventListener(
   "TalosPowersContentDumpConsole",
-  e => {
+  () => {
     var messages;
     try {
       messages = Services.console.getMessageArray();
@@ -147,7 +147,7 @@ addEventListener(
  */
 addEventListener(
   "TalosPowersWebRenderCapture",
-  e => {
+  () => {
     if (content && content.windowUtils) {
       content.windowUtils.wrCapture();
     } else {
diff --git a/testing/talos/talos/tests/cpstartup/extension/api.js b/testing/talos/talos/tests/cpstartup/extension/api.js
index 8b5e7438ce1e..7a881ef66f21 100644
--- a/testing/talos/talos/tests/cpstartup/extension/api.js
+++ b/testing/talos/talos/tests/cpstartup/extension/api.js
@@ -116,7 +116,7 @@ this.cpstartup = class extends ExtensionAPI {
   removeTab(tab) {
     return new Promise(resolve => {
       let browser = tab.linkedBrowser;
-      let observer = (subject, topic, data) => {
+      let observer = subject => {
         if (subject === browser) {
           Services.obs.removeObserver(observer, BROWSER_FLUSH_TOPIC);
           resolve();
diff --git a/testing/talos/talos/tests/cpstartup/extension/framescript.js b/testing/talos/talos/tests/cpstartup/extension/framescript.js
index 972f9388815c..ad09e24cb1ca 100644
--- a/testing/talos/talos/tests/cpstartup/extension/framescript.js
+++ b/testing/talos/talos/tests/cpstartup/extension/framescript.js
@@ -7,7 +7,7 @@
 
   addEventListener(
     "CPStartup:Ping",
-    e => {
+    () => {
       let evt = new content.CustomEvent("CPStartup:Pong", { bubbles: true });
       content.dispatchEvent(evt);
     },
diff --git a/testing/talos/talos/tests/devtools/addon/content/actors/DampLoadChild.sys.mjs b/testing/talos/talos/tests/devtools/addon/content/actors/DampLoadChild.sys.mjs
index 1f7db91c4bb8..5ef462c7f8a8 100644
--- a/testing/talos/talos/tests/devtools/addon/content/actors/DampLoadChild.sys.mjs
+++ b/testing/talos/talos/tests/devtools/addon/content/actors/DampLoadChild.sys.mjs
@@ -5,7 +5,7 @@
 /* eslint-disable no-restricted-globals */
 
 export class DampLoadChild extends JSWindowActorChild {
-  handleEvent(evt) {
+  handleEvent() {
     this.sendAsyncMessage("DampLoadChild:PageShow", {
       browsingContext: this.browsingContext,
     });
diff --git a/testing/talos/talos/tests/devtools/addon/content/damp.js b/testing/talos/talos/tests/devtools/addon/content/damp.js
index 9f710763d14f..cad8c8433d20 100644
--- a/testing/talos/talos/tests/devtools/addon/content/damp.js
+++ b/testing/talos/talos/tests/devtools/addon/content/damp.js
@@ -216,7 +216,7 @@ Damp.prototype = {
     return tab;
   },
 
-  async testTeardown(url) {
+  async testTeardown() {
     // Disable closing animation to avoid intermittents and prevent having to wait for
     // animation's end. (See bug 1480953)
     this._win.gBrowser.removeCurrentTab({ animate: false });
diff --git a/testing/talos/talos/tests/devtools/addon/content/tests/debugger/custom.js b/testing/talos/talos/tests/devtools/addon/content/tests/debugger/custom.js
index d0815bd5149d..3f2f8bc6e28a 100644
--- a/testing/talos/talos/tests/devtools/addon/content/tests/debugger/custom.js
+++ b/testing/talos/talos/tests/devtools/addon/content/tests/debugger/custom.js
@@ -145,7 +145,7 @@ async function stepDebuggerAndLog(dbg, tab, testFunction) {
   }
 }
 
-async function testProjectSearch(dbg, tab) {
+async function testProjectSearch(dbg) {
   dump("Executing project search\n");
   const test = runTest(`custom.jsdebugger.project-search.DAMP`);
   const firstSearchResultTest = runTest(
@@ -216,7 +216,7 @@ async function testPreview(dbg, tab, testFunction) {
   await garbageCollect();
 }
 
-async function testOpeningLargeMinifiedFile(dbg, tab) {
+async function testOpeningLargeMinifiedFile(dbg) {
   const fileFirstMinifiedChars = `(()=>{var e,t,n,r,o={82603`;
 
   dump("Open minified.js (large minified file)\n");
diff --git a/testing/talos/talos/tests/devtools/addon/content/tests/debugger/debugger-helpers.js b/testing/talos/talos/tests/devtools/addon/content/tests/debugger/debugger-helpers.js
index ee1e52ed0091..53ad52e65e8e 100644
--- a/testing/talos/talos/tests/devtools/addon/content/tests/debugger/debugger-helpers.js
+++ b/testing/talos/talos/tests/devtools/addon/content/tests/debugger/debugger-helpers.js
@@ -352,13 +352,7 @@ async function removeBreakpoints(dbg) {
 }
 exports.removeBreakpoints = removeBreakpoints;
 
-async function pauseDebugger(
-  dbg,
-  tab,
-  testFunction,
-  { line, file },
-  pauseOptions
-) {
+async function pauseDebugger(dbg, tab, testFunction, { line, file }) {
   const { getSelectedLocation, isMapScopesEnabled } = dbg.selectors;
 
   const state = dbg.store.getState();
diff --git a/testing/talos/talos/tests/devtools/addon/content/tests/head.js b/testing/talos/talos/tests/devtools/addon/content/tests/head.js
index 559ca21d2d3c..910cbe1e3003 100644
--- a/testing/talos/talos/tests/devtools/addon/content/tests/head.js
+++ b/testing/talos/talos/tests/devtools/addon/content/tests/head.js
@@ -114,7 +114,7 @@ function waitForDOMPredicate(
     return Promise.resolve(rv);
   }
   return new Promise(resolve => {
-    const observer = new target.ownerGlobal.MutationObserver(mutations => {
+    const observer = new target.ownerGlobal.MutationObserver(() => {
       rv = predicate();
       if (rv) {
         resolve(rv);
diff --git a/testing/talos/talos/tests/devtools/addon/content/tests/netmonitor/netmonitor-helpers.js b/testing/talos/talos/tests/devtools/addon/content/tests/netmonitor/netmonitor-helpers.js
index 4b860bfd1bd3..f981c399f620 100644
--- a/testing/talos/talos/tests/devtools/addon/content/tests/netmonitor/netmonitor-helpers.js
+++ b/testing/talos/talos/tests/devtools/addon/content/tests/netmonitor/netmonitor-helpers.js
@@ -45,7 +45,7 @@ async function waitForAllRequestsFinished(
     let payloadReady = 0;
     let resolveWithLessThanMaxRequestsTimer = null;
 
-    function onPayloadReady(_, id) {
+    function onPayloadReady() {
       payloadReady++;
       dump(`Waiting for ${maxExpectedRequests - payloadReady} requests\n`);
       maybeResolve();
diff --git a/testing/talos/talos/tests/devtools/addon/content/tests/server/actor.js b/testing/talos/talos/tests/devtools/addon/content/tests/server/actor.js
index 7180365c4040..e27059d4200b 100644
--- a/testing/talos/talos/tests/devtools/addon/content/tests/server/actor.js
+++ b/testing/talos/talos/tests/devtools/addon/content/tests/server/actor.js
@@ -12,7 +12,7 @@ class DampTestActor extends Actor {
     super(conn, dampTestSpec);
   }
 
-  testMethod(arg, { option }, arraySize) {
+  testMethod(arg, { option }) {
     // Emit an event with second argument's option.
     this.emit("testEvent", option);
 
diff --git a/testing/talos/talos/tests/devtools/addon/content/tests/server/protocol.js b/testing/talos/talos/tests/devtools/addon/content/tests/server/protocol.js
index 7cbd58d9dd02..8aae57618aa9 100644
--- a/testing/talos/talos/tests/devtools/addon/content/tests/server/protocol.js
+++ b/testing/talos/talos/tests/devtools/addon/content/tests/server/protocol.js
@@ -67,7 +67,7 @@ module.exports = async function () {
     bigObject["attribute-" + i] = bigString;
   }
 
-  let bigArray = Array.from({ length: ARRAY_SIZE }, (_, i) => bigObject);
+  let bigArray = Array.from({ length: ARRAY_SIZE }, _ => bigObject);
 
   // Open against options to avoid noise from tools
   let toolbox = await openToolbox("options");
diff --git a/testing/talos/talos/tests/devtools/addon/content/tests/toolbox/panels-in-background.js b/testing/talos/talos/tests/devtools/addon/content/tests/toolbox/panels-in-background.js
index 852e65467b88..7f27560a85c7 100644
--- a/testing/talos/talos/tests/devtools/addon/content/tests/toolbox/panels-in-background.js
+++ b/testing/talos/talos/tests/devtools/addon/content/tests/toolbox/panels-in-background.js
@@ -40,7 +40,7 @@ function waitForPayload(count, panelWin) {
   return new Promise(resolve => {
     let payloadReady = 0;
 
-    function onPayloadReady(_, id) {
+    function onPayloadReady() {
       payloadReady++;
       maybeResolve();
     }
diff --git a/testing/talos/talos/tests/tabpaint/api.js b/testing/talos/talos/tests/tabpaint/api.js
index abf64c6fe421..990267bdc2d9 100644
--- a/testing/talos/talos/tests/tabpaint/api.js
+++ b/testing/talos/talos/tests/tabpaint/api.js
@@ -160,7 +160,7 @@ this.tabpaint = class extends ExtensionAPI {
    *         Resolves once the tab has been fully removed. Resolves
    *         with the time (in ms) it took to open the tab from content.
    */
-  async openTabFromContent(gBrowser) {
+  async openTabFromContent() {
     TalosParentProfiler.subtestStart("TabPaint Content Start");
     let start_time = Cu.now();
 
@@ -200,7 +200,7 @@ this.tabpaint = class extends ExtensionAPI {
     TalosParentProfiler.mark("Tabpaint: Remove Tab");
     return new Promise(resolve => {
       let browser = tab.linkedBrowser;
-      let observer = (subject, topic, data) => {
+      let observer = subject => {
         if (subject === browser) {
           Services.obs.removeObserver(observer, BROWSER_FLUSH_TOPIC);
           resolve();
diff --git a/testing/talos/talos/tests/tabpaint/framescript.js b/testing/talos/talos/tests/tabpaint/framescript.js
index 2d4d7149fc08..3e209205b824 100644
--- a/testing/talos/talos/tests/tabpaint/framescript.js
+++ b/testing/talos/talos/tests/tabpaint/framescript.js
@@ -3,7 +3,7 @@
 (function () {
   addEventListener(
     "load",
-    loadevt => {
+    () => {
       if (!content.location.pathname.endsWith("target.html")) {
         return;
       }
@@ -62,7 +62,7 @@
 
   addEventListener(
     "TabPaint:Ping",
-    e => {
+    () => {
       let evt = new content.CustomEvent("TabPaint:Pong", { bubbles: true });
       content.dispatchEvent(evt);
     },
@@ -79,7 +79,7 @@
     true
   );
 
-  addMessageListener("TabPaint:OpenFromContent", msg => {
+  addMessageListener("TabPaint:OpenFromContent", () => {
     let evt = new content.CustomEvent("TabPaint:OpenFromContent", {
       bubbles: true,
     });
diff --git a/testing/talos/talos/tests/tabpaint/tabpaint.html b/testing/talos/talos/tests/tabpaint/tabpaint.html
index 0ab7242304da..b0329b5a9f74 100644
--- a/testing/talos/talos/tests/tabpaint/tabpaint.html
+++ b/testing/talos/talos/tests/tabpaint/tabpaint.html
@@ -13,7 +13,7 @@ function init() {
     tpRecordTime([fromParent, fromContent].join(","), 0, "tabpaint-from-parent, tabpaint-from-content");
   }, {once: true});
 
-  window.addEventListener("TabPaint:OpenFromContent", (event) => {
+  window.addEventListener("TabPaint:OpenFromContent", () => {
     let target = document.getElementById("target");
 
     //win.performance.now() + win.performance.timing.navigationStart gives the UNIX timestamp.
diff --git a/testing/talos/talos/tests/tabswitch/actors/TalosTabSwitchChild.sys.mjs b/testing/talos/talos/tests/tabswitch/actors/TalosTabSwitchChild.sys.mjs
index 5b27d677e321..bf456012f35b 100644
--- a/testing/talos/talos/tests/tabswitch/actors/TalosTabSwitchChild.sys.mjs
+++ b/testing/talos/talos/tests/tabswitch/actors/TalosTabSwitchChild.sys.mjs
@@ -21,7 +21,7 @@ export class TalosTabSwitchChild extends RemotePageChild {
     super.actorCreated();
   }
 
-  handleEvent(event) {}
+  handleEvent() {}
 
   receiveMessage(message) {
     if (message.name == "GarbageCollect") {
diff --git a/testing/talos/talos/tests/tabswitch/content/tabswitch-content-process.js b/testing/talos/talos/tests/tabswitch/content/tabswitch-content-process.js
index 53c493595308..b0cc7f47e3b7 100644
--- a/testing/talos/talos/tests/tabswitch/content/tabswitch-content-process.js
+++ b/testing/talos/talos/tests/tabswitch/content/tabswitch-content-process.js
@@ -27,7 +27,7 @@ const TPSProcessScript = {
         chan.originalURI = aURI;
         return chan;
       }
-      getURIFlags(aURI) {
+      getURIFlags() {
         return (
           Ci.nsIAboutModule.ALLOW_SCRIPT |
           Ci.nsIAboutModule.URI_MUST_LOAD_IN_CHILD |
diff --git a/testing/talos/talos/tests/video/video_playback.html b/testing/talos/talos/tests/video/video_playback.html
index 99bec7c6c1f1..53922ba4d7b5 100644
--- a/testing/talos/talos/tests/video/video_playback.html
+++ b/testing/talos/talos/tests/video/video_playback.html
@@ -39,7 +39,7 @@ function content_focused() {
   runTest();
 }
 
-function fullscreen(event) {
+function fullscreen() {
   if ((document.fullscreenElement && document.fullscreenElement !== null) ||
       document.mozFullScreen) {
     startTest();
diff --git a/testing/talos/talos/tests/webgl/benchmarks/terrain/perftest.html b/testing/talos/talos/tests/webgl/benchmarks/terrain/perftest.html
index a4721625526b..fc78c1d75c3f 100644
--- a/testing/talos/talos/tests/webgl/benchmarks/terrain/perftest.html
+++ b/testing/talos/talos/tests/webgl/benchmarks/terrain/perftest.html
@@ -188,7 +188,7 @@
   const WARMUP_TIMESTAMPS = 30; // Must be at least 2
   const MEASURED_FRAMES   = 100;
 
-  var gDoneCallback = function placeholder(intervals) {};
+  var gDoneCallback = function placeholder() {};
   var gCurrentTimestamp = 0;
   var gResultTimestamps = [];
 
@@ -202,7 +202,7 @@
     });
   }
 
-  function draw(timestamp) {
+  function draw() {
     // It's possible that under some implementations (even if not our current one),
     // the rAF callback arg will be in some way "optimized", e.g. always point to the
     // estimated next vsync timestamp, in order to allow the callee to have less
diff --git a/testing/talos/talos/webextensions/dummy/background.js b/testing/talos/talos/webextensions/dummy/background.js
index 1b20f85cfa41..d04a71d6164c 100644
--- a/testing/talos/talos/webextensions/dummy/background.js
+++ b/testing/talos/talos/webextensions/dummy/background.js
@@ -19,7 +19,7 @@ browser.webRequest.onBeforeRequest.addListener(
 
     let filter = browser.webRequest.filterResponseData(details.requestId);
 
-    filter.onstop = event => {
+    filter.onstop = () => {
       filter.close();
     };
     filter.ondata = event => {
@@ -48,7 +48,7 @@ browser.webRequest.onHeadersReceived.addListener(
   ["blocking", "responseHeaders"]
 );
 
-browser.webRequest.onErrorOccurred.addListener(details => {}, {
+browser.webRequest.onErrorOccurred.addListener(() => {}, {
   urls: ["https://*/*", "http://*/*"],
 });
 
@@ -73,7 +73,7 @@ browser.tabs.onUpdated.addListener((tabId, changed, tab) => {
   });
 });
 
-browser.tabs.onActivated.addListener(({ tabId, windowId }) => {
+browser.tabs.onActivated.addListener(({ tabId }) => {
   browser.pageAction.show(tabId);
 });
 
@@ -81,8 +81,8 @@ browser.tabs.onCreated.addListener(tab => {
   browser.pageAction.show(tab.id);
 });
 
-browser.tabs.onRemoved.addListener((tabId, removeInfo) => {});
+browser.tabs.onRemoved.addListener(() => {});
 
-browser.tabs.onAttached.addListener((tabId, attachInfo) => {});
+browser.tabs.onAttached.addListener(() => {});
 
-browser.tabs.onDetached.addListener((tabId, detachInfo) => {});
+browser.tabs.onDetached.addListener(() => {});
diff --git a/testing/xpcshell/head.js b/testing/xpcshell/head.js
index a487c481ef6b..d3b19719abeb 100644
--- a/testing/xpcshell/head.js
+++ b/testing/xpcshell/head.js
@@ -434,7 +434,7 @@ function _setupDevToolsServer(breakpointFiles, callback) {
     // Or when devtools are destroyed and we should stop observing.
     "xpcshell-test-devtools-shutdown",
   ];
-  let observe = function (subject, topic, data) {
+  let observe = function (subject, topic) {
     if (topic === "devtools-thread-ready") {
       const threadActor = subject.wrappedJSObject;
       threadActor.setBreakpointOnLoad(breakpointFiles);
@@ -745,7 +745,7 @@ function _execute_test() {
  * @param aFiles Array of files to load.
  */
 function _load_files(aFiles) {
-  function load_file(element, index, array) {
+  function load_file(element) {
     try {
       let startTime = Cu.now();
       load(element);
@@ -1829,7 +1829,7 @@ function run_next_test() {
   }
 
   function frontLoadSetups() {
-    _gTests.sort(([propsA, funcA], [propsB, funcB]) => {
+    _gTests.sort(([propsA], [propsB]) => {
       if (propsB.isSetup === propsA.isSetup) {
         return 0;
       }
diff --git a/testing/xpcshell/moz-http2/moz-http2.js b/testing/xpcshell/moz-http2/moz-http2.js
index f44421b029c3..c50001c3d2bc 100644
--- a/testing/xpcshell/moz-http2/moz-http2.js
+++ b/testing/xpcshell/moz-http2/moz-http2.js
@@ -56,7 +56,7 @@ var framer_module = node_http2_root + "/lib/protocol/framer";
 var http2_framer = require(framer_module);
 var Serializer = http2_framer.Serializer;
 var originalTransform = Serializer.prototype._transform;
-var newTransform = function (frame, encoding, done) {
+var newTransform = function (frame) {
   if (frame.type == "DATA") {
     // Insert our empty DATA frame
     const emptyFrame = {};
@@ -1757,7 +1757,7 @@ server.on("connection", function (socket) {
   });
 });
 
-server.on("connect", function (req, clientSocket, head) {
+server.on("connect", function (req, clientSocket) {
   clientSocket.write(
     "HTTP/1.1 404 Not Found\r\nProxy-agent: Node.js-Proxy\r\n\r\n"
   );