diff --git a/.eslintrc.js b/.eslintrc.js
index 76aedbcc8995..35003e943c78 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -482,7 +482,6 @@ module.exports = {
         "layout/**",
         "netwerk/**",
         "storage/test/unit/test_vacuum.js",
-        "testing/**",
         "tools/**",
         "widget/tests/test_assign_event_data.html",
       ],
diff --git a/testing/mochitest/browser-test.js b/testing/mochitest/browser-test.js
index f99f8524be01..dbe38513832c 100644
--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -593,7 +593,7 @@ Tester.prototype = {
       // Run cleanup functions for the current test before moving on to the
       // next one.
       let testScope = this.currentTest.scope;
-      while (testScope.__cleanupFunctions.length > 0) {
+      while (testScope.__cleanupFunctions.length) {
         let func = testScope.__cleanupFunctions.shift();
         try {
           let result = await func.apply(testScope);
diff --git a/testing/mochitest/manifestLibrary.js b/testing/mochitest/manifestLibrary.js
index 55a0a303275b..c9417f03d1a5 100644
--- a/testing/mochitest/manifestLibrary.js
+++ b/testing/mochitest/manifestLibrary.js
@@ -49,7 +49,7 @@ function parseTestManifest(testManifest, params, callback) {
       });
     }
   }
-  if (paths.length > 0) {
+  if (paths.length) {
     callback(paths);
   } else {
     callback(links);
diff --git a/testing/mochitest/tests/SimpleTest/MemoryStats.js b/testing/mochitest/tests/SimpleTest/MemoryStats.js
index cedd8fe640f9..eb319b2e76d7 100644
--- a/testing/mochitest/tests/SimpleTest/MemoryStats.js
+++ b/testing/mochitest/tests/SimpleTest/MemoryStats.js
@@ -94,7 +94,7 @@ MemoryStats.dump = function(
       );
     }
   }
-  if (statMessage.length > 0) {
+  if (statMessage.length) {
     info("MEMORY STAT" + statMessage);
   }
 
diff --git a/testing/mochitest/tests/SimpleTest/SimpleTest.js b/testing/mochitest/tests/SimpleTest/SimpleTest.js
index eb0dcc769271..94dd508f8c15 100644
--- a/testing/mochitest/tests/SimpleTest/SimpleTest.js
+++ b/testing/mochitest/tests/SimpleTest/SimpleTest.js
@@ -252,7 +252,7 @@ if (typeof partial == "undefined") {
       args.push(arguments[i]);
     }
     return function() {
-      if (arguments.length > 0) {
+      if (arguments.length) {
         for (let i = 1; i < arguments.length; i++) {
           args.push(arguments[i]);
         }
@@ -1409,7 +1409,7 @@ SimpleTest.finish = function() {
         "expectUncaughtException was called but no uncaught exception was detected!"
       );
     }
-    if (SimpleTest._tests.length == 0) {
+    if (!SimpleTest._tests.length) {
       SimpleTest.ok(
         false,
         "[SimpleTest.finish()] No checks actually run. " +
@@ -1429,7 +1429,7 @@ SimpleTest.finish = function() {
           "This test is expected to leave a service worker registered"
         );
       }
-    } else if (workers.length > 0) {
+    } else if (workers.length) {
       let FULL_PROFILE_WORKERS_TO_IGNORE = [];
       if (parentRunner.conditionedProfile) {
         // Full profile has service workers in the profile, without clearing the profile
diff --git a/testing/mochitest/tests/SimpleTest/TestRunner.js b/testing/mochitest/tests/SimpleTest/TestRunner.js
index b850fce030de..40b36a839180 100644
--- a/testing/mochitest/tests/SimpleTest/TestRunner.js
+++ b/testing/mochitest/tests/SimpleTest/TestRunner.js
@@ -942,7 +942,7 @@ TestRunner.displayLoopErrors = function(tableName, tests) {
   if (TestRunner.countResults(tests).notOK > 0) {
     var table = $(tableName);
     var curtest;
-    if (table.rows.length == 0) {
+    if (!table.rows.length) {
       //if table headers are not yet generated, make them
       var row = table.insertRow(table.rows.length);
       var cell = row.insertCell(0);
diff --git a/testing/mochitest/tests/SimpleTest/paint_listener.js b/testing/mochitest/tests/SimpleTest/paint_listener.js
index 4f0388c2c323..475ac28fc9f0 100644
--- a/testing/mochitest/tests/SimpleTest/paint_listener.js
+++ b/testing/mochitest/tests/SimpleTest/paint_listener.js
@@ -40,7 +40,7 @@
     if (debug) {
       dump("Dispatching " + onpaint.length + " onpaint listeners\n");
     }
-    while (onpaint.length > 0) {
+    while (onpaint.length) {
       window.setTimeout(onpaint.pop(), 0);
     }
   }
diff --git a/testing/mochitest/tests/SimpleTest/setup.js b/testing/mochitest/tests/SimpleTest/setup.js
index 836c899facc3..6d8b3bf9dc9a 100644
--- a/testing/mochitest/tests/SimpleTest/setup.js
+++ b/testing/mochitest/tests/SimpleTest/setup.js
@@ -31,7 +31,7 @@ if (!("SpecialPowers" in window)) {
 function parseQueryString(encodedString, useArrays) {
   // strip a leading '?' from the encoded string
   var qstr =
-    encodedString.length > 0 && encodedString[0] == "?"
+    encodedString.length && encodedString[0] == "?"
       ? encodedString.substring(1)
       : encodedString;
   var pairs = qstr.replace(/\+/g, "%20").split(/(\&amp\;|\&\#38\;|\&#x26;|\&)/);
@@ -296,7 +296,7 @@ function hookup() {
 }
 
 function hookupTests(testList) {
-  if (testList.length > 0) {
+  if (testList.length) {
     gTestList = testList;
   } else {
     gTestList = [];
diff --git a/testing/raptor/webext/raptor/runner.js b/testing/raptor/webext/raptor/runner.js
index 2973e246c284..643bb68ab51f 100644
--- a/testing/raptor/webext/raptor/runner.js
+++ b/testing/raptor/webext/raptor/runner.js
@@ -233,7 +233,7 @@ async function closeTab(tabId) {
 
 async function getCurrentTabId() {
   const tabs = await queryForTabs({ currentWindow: true, active: true });
-  if (tabs.length == 0) {
+  if (!tabs.length) {
     throw new Error("No active tab has been found.");
   }
 
@@ -594,7 +594,7 @@ function resultListener(request, sender, sendResponse) {
           const index = pendingHeroes.indexOf(_found);
           if (index > -1) {
             pendingHeroes.splice(index, 1);
-            if (pendingHeroes.length == 0) {
+            if (!pendingHeroes.length) {
               raptorLog("measured all expected hero elements");
               isHeroPending = false;
             }
diff --git a/testing/specialpowers/content/SpecialPowersParent.jsm b/testing/specialpowers/content/SpecialPowersParent.jsm
index 7aa5de5ecd59..08b87878559a 100644
--- a/testing/specialpowers/content/SpecialPowersParent.jsm
+++ b/testing/specialpowers/content/SpecialPowersParent.jsm
@@ -287,7 +287,7 @@ class SpecialPowersParent extends JSWindowActorParent {
       return false;
     }
 
-    var success = aFilenames.length != 0;
+    var success = !!aFilenames.length;
     aFilenames.forEach(function(crashFilename) {
       var file = crashDumpDir.clone();
       file.append(crashFilename);
diff --git a/testing/talos/talos/pageloader/chrome/pageloader.js b/testing/talos/talos/pageloader/chrome/pageloader.js
index c81703122c44..382f932a75d0 100644
--- a/testing/talos/talos/pageloader/chrome/pageloader.js
+++ b/testing/talos/talos/pageloader/chrome/pageloader.js
@@ -173,7 +173,7 @@ async function plInit() {
       plStop(true);
     }
 
-    if (pages.length == 0) {
+    if (!pages.length) {
       dumpLine("tp: no pages to test, quitting");
       plStop(true);
     }
diff --git a/testing/talos/talos/tests/tabswitch/api.js b/testing/talos/talos/tests/tabswitch/api.js
index c20f4cd36706..d21e64bb1d53 100644
--- a/testing/talos/talos/tests/tabswitch/api.js
+++ b/testing/talos/talos/tests/tabswitch/api.js
@@ -230,7 +230,7 @@ async function test(window) {
   } catch (ex) {
     /* error condition handled below */
   }
-  if (!testURLs || testURLs.length == 0) {
+  if (!testURLs || !testURLs.length) {
     dump(
       "no tabs to test, 'addon.test.tabswitch.urlfile' pref isn't set to page set path\n"
     );
diff --git a/testing/xpcshell/head.js b/testing/xpcshell/head.js
index ddc0047872aa..c9024f09f8b9 100644
--- a/testing/xpcshell/head.js
+++ b/testing/xpcshell/head.js
@@ -651,7 +651,7 @@ function _execute_test() {
     });
   };
 
-  let complete = _cleanupFunctions.length == 0;
+  let complete = !_cleanupFunctions.length;
   let cleanupStartTime = complete ? 0 : Cu.now();
   (async () => {
     for (let func of _cleanupFunctions.reverse()) {
diff --git a/testing/xpcshell/moz-http2/moz-http2.js b/testing/xpcshell/moz-http2/moz-http2.js
index 43379ef4fcd9..9a431e7b8aa2 100644
--- a/testing/xpcshell/moz-http2/moz-http2.js
+++ b/testing/xpcshell/moz-http2/moz-http2.js
@@ -312,7 +312,7 @@ function handleRequest(req, res) {
 
   function responseType(packet, responseIP) {
     if (
-      packet.questions.length > 0 &&
+      !!packet.questions.length &&
       packet.questions[0].name == "confirm.example.com" &&
       packet.questions[0].type == "NS"
     ) {
@@ -343,24 +343,18 @@ function handleRequest(req, res) {
 
   function createDNSAnswer(response, packet, responseIP, requestPayload) {
     // This shuts down the connection so we can test if the client reconnects
-    if (
-      packet.questions.length > 0 &&
-      packet.questions[0].name == "closeme.com"
-    ) {
+    if (packet.questions.length && packet.questions[0].name == "closeme.com") {
       response.stream.connection.close("INTERNAL_ERROR", response.stream.id);
       return null;
     }
 
-    if (
-      packet.questions.length > 0 &&
-      packet.questions[0].name.endsWith(".pd")
-    ) {
+    if (packet.questions.length && packet.questions[0].name.endsWith(".pd")) {
       // Bug 1543811: test edns padding extension. Return whether padding was
       // included via the first half of the ip address (1.1 vs 2.2) and the
       // size of the request in the second half of the ip address allowing to
       // verify that the correct amount of padding was added.
       if (
-        packet.additionals.length > 0 &&
+        !!packet.additionals.length &&
         packet.additionals[0].type == "OPT" &&
         packet.additionals[0].options.some(o => o.type === "PADDING")
       ) {
@@ -394,7 +388,7 @@ function handleRequest(req, res) {
 
     function responseData() {
       if (
-        packet.questions.length > 0 &&
+        !!packet.questions.length &&
         packet.questions[0].name == "confirm.example.com" &&
         packet.questions[0].type == "NS"
       ) {