diff --git a/.eslintignore b/.eslintignore
index 0d6164650788..60ad59e59a46 100644
--- a/.eslintignore
+++ b/.eslintignore
@@ -13,8 +13,6 @@ obj*/**
 
 # dom/ exclusions which should be removed (aka ESLint enabled)
 dom/base/*.*
-dom/base/test/*.*
-dom/base/test/jsmodules/**
 dom/media/test/**
 !dom/media/test/marionette/yttest/*.js
 dom/promise/**
@@ -169,6 +167,7 @@ dom/svg/test/test_nonAnimStrings.xhtml
 dom/svg/test/test_SVG_namespace_ids.html
 
 # Strange encodings
+dom/base/test/file_bug687859-16.js
 dom/encoding/test/test_utf16_files.html
 dom/encoding/test/file_utf16_be_bom.js
 dom/encoding/test/file_utf16_le_bom.js
@@ -179,6 +178,12 @@ dom/html/test/test_bug677658.html
 
 # Intentional broken files
 dom/base/test/file_js_cache_syntax_error.js
+dom/base/test/jsmodules/test_syntaxError.html
+dom/base/test/jsmodules/test_syntaxErrorAsync.html
+dom/base/test/jsmodules/module_badSyntax.js
+dom/base/test/jsmodules/test_syntaxErrorInline.html
+dom/base/test/jsmodules/test_syntaxErrorInlineAsync.html
+dom/base/test/test_bug687859.html
 dom/media/tests/mochitest/identity/idp-bad.js
 dom/serviceworkers/test/file_js_cache_syntax_error.js
 dom/serviceworkers/test/parse_error_worker.js
diff --git a/dom/base/test/browser_aboutnewtab_process_selection.js b/dom/base/test/browser_aboutnewtab_process_selection.js
index 5c426343003d..d8d88f42e6a2 100644
--- a/dom/base/test/browser_aboutnewtab_process_selection.js
+++ b/dom/base/test/browser_aboutnewtab_process_selection.js
@@ -62,10 +62,12 @@ add_task(async function(){
 
   // Navigate to a content page from the child side.
   await BrowserTestUtils.switchTab(gBrowser, tab1);
+  /* eslint-disable no-shadow */
   await ContentTask.spawn(tab1.linkedBrowser, null, async function() {
     const TEST_URL = "http://www.example.com/browser/dom/base/test/dummy.html";
     content.location.href = TEST_URL;
   });
+  /* eslint-enable no-shadow */
   await BrowserTestUtils.browserLoaded(tab1.linkedBrowser, false, TEST_URL);
   is(ppmm.childCount, originalChildCount + 2,
      "Navigating away from the preloaded browser (child side) should create a new content process.")
@@ -106,4 +108,3 @@ add_task(async function preloaded_state_attribute() {
   BrowserTestUtils.removeTab(gBrowser.selectedTab);
   NewTabPagePreloading.removePreloadedBrowser(window);
 });
-
diff --git a/dom/base/test/browser_force_process_selector.js b/dom/base/test/browser_force_process_selector.js
index b0b3abb93451..0ec280c97f83 100644
--- a/dom/base/test/browser_force_process_selector.js
+++ b/dom/base/test/browser_force_process_selector.js
@@ -20,7 +20,7 @@ async function spawnNewAndTest(recur, pids) {
         };
         Services.obs.addObserver(observer, CONTENT_CREATED);
 
-        await BrowserTestUtils.withNewTab({ gBrowser, url: "about:blank" }, function(browser) {
+        await BrowserTestUtils.withNewTab({ gBrowser, url: "about:blank" }, function() {
           // If this new tab caused us to create a new process, the ok(false)
           // should have already happened. Therefore, if we get here, we've
           // passed. Simply remove the observer.
diff --git a/dom/base/test/browser_inputStream_structuredClone.js b/dom/base/test/browser_inputStream_structuredClone.js
index d0a772c3f521..de6ec0e733fa 100644
--- a/dom/base/test/browser_inputStream_structuredClone.js
+++ b/dom/base/test/browser_inputStream_structuredClone.js
@@ -22,6 +22,7 @@ async function runTest(input, url) {
 
   is(data.inputStream.available(), input.length, "The length of the inputStream matches: " + input.length);
 
+  /* eslint-disable no-shadow */
   let dataBack = await ContentTask.spawn(browser, data, function(data) {
     let dataBack = {
       inputStream: data.inputStream,
@@ -34,6 +35,8 @@ async function runTest(input, url) {
 
     return dataBack;
   });
+  /* eslint-enable no-shadow */
+
 
   ok(dataBack.check, "The inputStream is a nsIInputStream also on content.");
   ok(data.inputStream instanceof Ci.nsIInputStream, "The original object was an inputStream");
diff --git a/dom/base/test/browser_state_notifications.js b/dom/base/test/browser_state_notifications.js
index 865bc3db73c2..16576f561e35 100644
--- a/dom/base/test/browser_state_notifications.js
+++ b/dom/base/test/browser_state_notifications.js
@@ -50,6 +50,7 @@ const receive = (topic, p, syncCallback) => {
         }
       }
     };
+    // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
     const id = setTimeout(timeout, 90000);
     addObserver(observer, topic, false);
     queue.push(topic, resolve);
@@ -59,6 +60,7 @@ receive.queue = [];
 
 const openTab = uri => gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, uri);
 
+// eslint-disable-next-line mozilla/no-arbitrary-setTimeout
 const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
 
 const isData = document => document.URL.startsWith("data:");
diff --git a/dom/base/test/file_bug416317.xhtml b/dom/base/test/file_bug416317.xhtml
index 9c6d219c60b7..b365e364ac2f 100644
--- a/dom/base/test/file_bug416317.xhtml
+++ b/dom/base/test/file_bug416317.xhtml
@@ -226,6 +226,8 @@
     .bar. { background: red; color: yellow; }
   </span>
   <script><![CDATA[
+  /* eslint-disable no-shadow */
+
   window.onload = function() {
     doTest();
   }
@@ -242,13 +244,13 @@
     var cache = {};
 
     var css = document.getElementById("test").firstChild.nodeValue.split("\n");
-    for ( var i = 0; i < css.length; i++ ) {
+    for ( let i = 0; i < css.length; i++ ) {
       css[i] = css[i].replace(/\/\*.*?\*\//g, "")
         .replace(/^\s*|\s*$/g, "").split(/\s*{/);
     }
 
     var ecss = document.getElementById("error").firstChild.nodeValue.split("\n");
-    for ( var i = 0; i < ecss.length; i++ ) {
+    for ( let i = 0; i < ecss.length; i++ ) {
       ecss[i] = ecss[i].replace(/\/\*.*?\*\//g, "")
         .replace(/^\s*|\s*$/g, "").split(/\s*{/);
     }
@@ -290,14 +292,14 @@
     cacheCheck( "Fragment", fragment );
 
     root.remove( );
-    
+
     interfaceCheck(document, "Document");
     runTest( css, "Document", document, true );
     check( "Document", document, true, false );
     runTest( ecss, "SyntaxError: Document", document, false );
     jqTests("Document", document, "querySelectorAll");
     cacheCheck( "Document", document );
-    
+
     done();
 
     function interfaceCheck(obj, type){
@@ -320,7 +322,7 @@
         li.appendChild( document.createTextNode( ": " + passed + " passed, " + (tests - passed) + " failed" ) );
         r.appendChild( li );
 
-        for ( var i = 0; i < results.length; i++ ) {
+        for ( let i = 0; i < results.length; i++ ) {
               var li = document.createElement("li");
               var span = document.createElement("span");
               span.style.color = (results[i][0] === "FAIL" ? "red" : "green");
@@ -404,14 +406,14 @@
       } catch(e){ pass = true; }
       assert( pass, type + ".querySelector no value" );
 
-      for ( var i = 0; i < css.length; i++ ) {
+      for ( let i = 0; i < css.length; i++ ) {
         var test = css[i];
         if ( test.length == 2 ) {
           var query = test[0], color = test[1].match(/: ([^\s;]+)/)[1];
-  
+
           try {
             var found = root.querySelectorAll(query);
-  
+
             for ( var f = 0; f < found.length; f++ ) {
               found[f].style.backgroundColor = color;
             }
@@ -563,7 +565,7 @@
                       (e.name == "SyntaxError" && e.code == DOMException.SYNTAX_ERR));
             }
 
-            assert( pass, type + ": " + name + " Bad Resolver #" + (i+1) + " (" + nq + ")" + 
+            assert( pass, type + ": " + name + " Bad Resolver #" + (i+1) + " (" + nq + ")" +
               (pass ? "" : " Expected: " + extra(ids) + " Received: " + extra(results)) );
           }
         } else {
@@ -575,7 +577,7 @@
           } catch(e) {
             pass = e.name == "SyntaxError" && e.code == DOMException.SYNTAX_ERR;
           }
-  
+
           assert( pass, type + ": " + name + " (" + nq + ")" +
             (pass ? "" : " Expected: " + extra(ids) + " Received: " + extra(results)) );
 
@@ -588,7 +590,7 @@
 
         function hasPassed(results, ids){
           var pass = (results && results.length == ids.length) || (!results && !ids);
-  
+
           if ( ids && results ) {
             for ( var i = 0; ids && i < ids.length; i++ ) {
               if ( ids[i] !== results[i].getAttribute("id") ) {
@@ -609,7 +611,7 @@
               extra += (extra.length > 2 ? "," : "") + "'" + (results[i].id || results[i]) + "'";
             }
           }
-  
+
           extra += "]";
           return extra;
         }
@@ -672,7 +674,7 @@
       var param = query("#object1 param");
       assert( param && param.length == 2, type + ": Object/param as context" );
 
-      var l = query("#length");  
+      var l = query("#length");
       assert( l && l.length, type + ': &lt;input name="length"&gt; cannot be found under IE' );
       var lin = query("#lengthtest input");
       assert( lin && lin.length, type + ': &lt;input name="length"&gt; cannot be found under IE' );
@@ -695,18 +697,18 @@
       t( "Multiple ID selectors using UTF8", "#台北Táiběi, #台北", ["台北Táiběi","台北"] );
       t( "Descendant ID selector using UTF8", "div #台北", ["台北"] );
       t( "Child ID selector using UTF8", "form > #台北", ["台北"] );
-  
+
       t( "Escaped ID", "#foo\\:bar", ["foo:bar"] );
       t( "Escaped ID", "#test\\.foo\\[5\\]bar", ["test.foo[5]bar"] );
       t( "Descendant escaped ID", "div #foo\\:bar", ["foo:bar"] );
       t( "Descendant escaped ID", "div #test\\.foo\\[5\\]bar", ["test.foo[5]bar"] );
       t( "Child escaped ID", "form > #foo\\:bar", ["foo:bar"] );
       t( "Child escaped ID", "form > #test\\.foo\\[5\\]bar", ["test.foo[5]bar"] );
-  
+
       t( "ID Selector, child ID present", "#form > #radio1", ["radio1"] ); // bug #267
       t( "ID Selector, not an ancestor ID", "#form #first", [] );
       t( "ID Selector, not a child ID", "#form > #option1a", [] );
-      
+
       t( "All Children of ID", "#foo > *", ["sndp", "en", "sap"] );
       t( "All Children of ID with no children", "#firstUL > *", [] );
 
@@ -718,14 +720,14 @@
       t( "Class Selector", ".blog.link", ["simon"] );
       t( "Class Selector w/ Element", "a.blog", ["mark","simon"] );
       t( "Parent Class Selector", "p .blog", ["mark","simon"] );
-  
+
       t( "Class selector using UTF8", ".台北Táiběi", ["utf8class1"] );
       t( "Class selector using UTF8", ".台北", ["utf8class1","utf8class2"] );
       t( "Class selector using UTF8", ".台北Táiběi.台北", ["utf8class1"] );
       t( "Class selector using UTF8", ".台北Táiběi, .台北", ["utf8class1","utf8class2"] );
       t( "Descendant class selector using UTF8", "div .台北Táiběi", ["utf8class1"] );
       t( "Child class selector using UTF8", "form > .台北Táiběi", ["utf8class1"] );
-  
+
       t( "Escaped Class", ".foo\\:bar", ["foo:bar"] );
       t( "Escaped Class", ".test\\.foo\\[5\\]bar", ["test.foo[5]bar"] );
       t( "Descendant scaped Class", "div .foo\\:bar", ["foo:bar"] );
@@ -756,13 +758,13 @@
       t( "Adjacent", "a+a", ["groups"] );
       t( "Adjacent", "p + p", ["ap","en","sap"] );
       t( "Comma, Child, and Adjacent", "a + a, code > a", ["groups","anchor1","anchor2"] );
-      
+
       t( "First Child", "p:first-child", ["firstp","sndp"] );
       t( "Nth Child", "p:nth-child(1)", ["firstp","sndp"] );
-      
+
       t( "Last Child", "p:last-child", ["sap"] );
       t( "Last Child", "a:last-child", ["simon1","anchor1","mark","yahoo","anchor2","simon"] );
-  
+
       t( "Nth-child", "#main form#form > *:nth-child(2)", ["text2"] );
       t( "Nth-child", "#main form#form > :nth-child(2)", ["text2"] );
 
@@ -788,27 +790,27 @@
       t( "Attribute Exists", "a[title]", ["google"] );
       t( "Attribute Exists", "*[title]", ["google"] );
       t( "Attribute Exists", "[title]", ["google"] );
-      
+
       t( "Attribute Equals", "a[rel='bookmark']", ["simon1"] );
       t( "Attribute Equals", 'a[rel="bookmark"]', ["simon1"] );
       t( "Attribute Equals", "a[rel=bookmark]", ["simon1"] );
       t( "Multiple Attribute Equals", "#form input[type='hidden'],#form input[type='radio']", ['radio1','radio2','hidden1'] );
       t( "Multiple Attribute Equals", "#form input[type=\"hidden\"],#form input[type='radio']", ['radio1','radio2','hidden1'] );
       t( "Multiple Attribute Equals", "#form input[type=hidden],#form input[type=radio]", ['radio1','radio2','hidden1'] );
-  
+
       t( "Attribute selector using UTF8", "span[lang=中文]", ["台北"] );
-  
+
       t( "Attribute Begins With", "a[href ^= 'http://www']", ["google","yahoo"] );
       t( "Attribute Ends With", "a[href $= 'org/']", ["mark"] );
       t( "Attribute Contains", "a[href *= 'google']", ["google","groups"] );
-  
+
       // t("Select options via [selected]", "#select1 option[selected]", ["option1a"] );
       t("Select options via [selected]", "#select1 option[selected]", [] );
       t("Select options via [selected]", "#select2 option[selected]", ["option2d"] );
       t("Select options via [selected]", "#select3 option[selected]", ["option3b", "option3c"] );
-  
+
       t( "Grouped Form Elements", "input[name='foo[bar]']", ["hidden2"] );
-  
+
       t( ":not() Existing attribute", "#form select:not([multiple])", ["select1", "select2"]);
       t( ":not() Equals attribute", "#form select:not([name=select1])", ["select2", "select3"]);
       t( ":not() Equals quoted attribute", "#form select:not([name='select1'])", ["select2", "select3"]);
@@ -1276,7 +1278,7 @@
   <h1 id="header">jQuery Test Suite</h1>
   <h2 id="banner"></h2>
   <h2 id="userAgent"></h2>
-  
+
   <!-- Test HTML -->
   <div id="nothiddendiv" style="height:1px;background:white;">
 
@@ -1293,9 +1295,9 @@
     <p id="firstp">See <a id="simon1" href="http://simon.incutio.com/archive/2003/03/25/#getElementsBySelector" rel="bookmark">this blog entry</a> for more information.</p>
 
     <p id="ap">
-      Here are some links in a normal paragraph: <a id="google" href="http://www.google.com/" title="Google!">Google</a>, 
-      <a id="groups" href="http://groups.google.com/">Google Groups</a>. 
-      This link has <code><a href="http://smin" id="anchor1">class="blog"</a></code>: 
+      Here are some links in a normal paragraph: <a id="google" href="http://www.google.com/" title="Google!">Google</a>,
+      <a id="groups" href="http://groups.google.com/">Google Groups</a>.
+      This link has <code><a href="http://smin" id="anchor1">class="blog"</a></code>:
       <a href="http://diveintomark.org/" class="blog" hreflang="en" id="mark">diveintomark</a>
 
     </p>
@@ -1322,14 +1324,14 @@
 
       <input type="hidden" name="hidden" id="hidden1"/>
       <input type="text" style="display:none;" name="foo[bar]" id="hidden2"/>
-      
+
       <input type="text" id="name" name="name" value="name" />
-      
+
       <button id="button" name="button">Button</button>
-      
+
       <textarea id="area1" maxlength="30">foobar</textarea>
 
-      
+
       <select name="select1" id="select1">
         <option id="option1a" class="emptyopt" value="">Nothing</option>
         <option id="option1b" value="1">1</option>
@@ -1350,20 +1352,20 @@
         <option id="option3c" selected="selected" value="2">2</option>
         <option id="option3d" value="3">3</option>
       </select>
-      
+
       <object id="object1" codebase="stupid">
         <param name="p1" value="x1" />
         <param name="p2" value="x2" />
 
       </object>
-      
+
       <span id="台北Táiběi"></span>
       <span id="台北" lang="中文"></span>
       <span id="utf8class1" class="台北Táiběi 台北"></span>
       <span id="utf8class2" class="台北"></span>
       <span id="foo:bar" class="foo:bar"></span>
       <span id="test.foo[5]bar" class="test.foo[5]bar"></span>
-      
+
       <foo_bar id="foobar">test element</foo_bar>
 
     </form>
@@ -1375,27 +1377,27 @@
     </form>
     <table id="table"></table>
 
-    
+
     <div id="fx-queue">
       <div id="fadein" class='chain test'>fadeIn<div>fadeIn</div></div>
       <div id="fadeout" class='chain test out'>fadeOut<div>fadeOut</div></div>
-      
+
       <div id="show" class='chain test'>show<div>show</div></div>
       <div id="hide" class='chain test out'>hide<div>hide</div></div>
 
-      
+
       <div id="togglein" class='chain test'>togglein<div>togglein</div></div>
       <div id="toggleout" class='chain test out'>toggleout<div>toggleout</div></div>
-    
-      
+
+
       <div id="slideup" class='chain test'>slideUp<div>slideUp</div></div>
       <div id="slidedown" class='chain test out'>slideDown<div>slideDown</div></div>
-      
+
       <div id="slidetogglein" class='chain test'>slideToggleIn<div>slideToggleIn</div></div>
 
       <div id="slidetoggleout" class='chain test out'>slideToggleOut<div>slideToggleOut</div></div>
     </div>
-    
+
     <div id="fx-tests"></div>
 
     <form id="testForm" action="#" method="get">
@@ -1462,7 +1464,7 @@ Z</textarea>
     </div>
   </div>
   </dl>
-  
+
   <ol id="tests"></ol>
 </div>
 <ol id="results"></ol>
diff --git a/dom/base/test/file_history_document_open.html b/dom/base/test/file_history_document_open.html
index 807b8d66bca8..b9f05f7c2c82 100644
--- a/dom/base/test/file_history_document_open.html
+++ b/dom/base/test/file_history_document_open.html
@@ -1 +1 @@
-<script>function f() { history.length; } window.onload = function() { var f = window.f; document.open(); document.close(); parent.continueTest(f); }</script>
+<script>function f() { history.length; } window.onload = function() { var func = window.f; document.open(); document.close(); parent.continueTest(func); }</script>
diff --git a/dom/base/test/jsmodules/.eslintrc.js b/dom/base/test/jsmodules/.eslintrc.js
new file mode 100644
index 000000000000..b6eb63514520
--- /dev/null
+++ b/dom/base/test/jsmodules/.eslintrc.js
@@ -0,0 +1,7 @@
+"use strict";
+
+module.exports = {
+  "parserOptions": {
+    "sourceType": "module",
+  }
+};
diff --git a/dom/base/test/referrer_helper.js b/dom/base/test/referrer_helper.js
index 8f1adb13660d..cd47b04e6950 100644
--- a/dom/base/test/referrer_helper.js
+++ b/dom/base/test/referrer_helper.js
@@ -1,3 +1,6 @@
+// This helper expects these globals to be defined.
+/* global PARAMS, SJS, testCases */
+
 /*
  * common functionality for iframe, anchor, and area referrer attribute tests
  */
@@ -80,22 +83,22 @@ var tests = (function*() {
     }
 
     var actions = testCases[j].ACTION;
-    var tests = testCases[j].TESTS;
+    var subTests = testCases[j].TESTS;
     for (var k = 0; k < actions.length; k++) {
       var actionString = actions[k];
-      for (var i = 0; i < tests.length; i++) {
+      for (var i = 0; i < subTests.length; i++) {
         yield resetState();
         var searchParams = new URLSearchParams();
         searchParams.append("ACTION", actionString);
-        searchParams.append("NAME", tests[i].NAME);
+        searchParams.append("NAME", subTests[i].NAME);
         for (var l of PARAMS) {
-          if (tests[i][l]) {
-            searchParams.append(l, tests[i][l]);
+          if (subTests[i][l]) {
+            searchParams.append(l, subTests[i][l]);
           }
         }
-        var schemeFrom = tests[i].SCHEME_FROM || "http";
+        var schemeFrom = subTests[i].SCHEME_FROM || "http";
         yield iframe.src = schemeFrom + SJS + searchParams.toString();
-        yield checkIndividualResults(tests[i].DESC, tests[i].RESULT, tests[i].NAME);
+        yield checkIndividualResults(subTests[i].DESC, subTests[i].RESULT, subTests[i].NAME);
       };
     };
   };
diff --git a/dom/base/test/test_NodeIterator_basics_filters.xhtml b/dom/base/test/test_NodeIterator_basics_filters.xhtml
index c9cca97157da..47504eebbe41 100644
--- a/dom/base/test/test_NodeIterator_basics_filters.xhtml
+++ b/dom/base/test/test_NodeIterator_basics_filters.xhtml
@@ -21,7 +21,7 @@
   function compare_arrays(e, f, label) {
     var length = (e.length > f.length) ? e.length : f.length;
     for (var i = 0; i < length; i += 1) {
-      if (e[i] > 0) 
+      if (e[i] > 0)
         is(f[i], e[i], label + " - index " + i + ": ");
       else
         todo_is(f[i], -e[i], label + " - index " + i + ": ");
@@ -110,17 +110,17 @@
   compare_arrays(expect_f, found, 'filtered backward');
 
   function checkBadFilter(method, n) {
-    var iterator =
+    var iter =
       document.createNodeIterator(document, NodeFilter.SHOW_ALL,
                                   function() {
                                     if (n < 0)
-                                      iterator.detach();
+                                      iter.detach();
                                     return NodeFilter.FILTER_ACCEPT;
                                   });
     while (--n >= 0)
-      iterator.nextNode();
+      iter.nextNode();
     try {
-      iterator[method]();
+      iter[method]();
       ok(true, "Able to call " + method + " on a NodeIterator after calling no-op detach()");
     } catch (x) { ok(false, x) }
   }
@@ -131,7 +131,7 @@
     // Implementing the scenario outlined in
     // http://bugzilla.mozilla.org/show_bug.cgi?id=552110#c4
 
-    var iterator = (function(filter) {
+    var iter = (function(filt) {
       var grandparent = document.createElement("div"),
           parent = document.createElement("span");
 
@@ -141,12 +141,12 @@
 
       return document.createNodeIterator(grandparent,
                                          NodeFilter.SHOW_ALL,
-                                         filter);
-    })(function filter(n) {
+                                         filt);
+    })(function(n) {
       if (n.nodeName != "img")
         return NodeFilter.FILTER_ACCEPT;
 
-      iterator.detach();
+      iter.detach();
 
       n.parentNode.remove();
       // Drop any node references passed into this function.
@@ -162,16 +162,16 @@
       return NodeFilter.FILTER_SKIP;
     });
 
-    is(iterator.nextNode().nodeName, "div",
-       "iterator.nextNode() returned the wrong node");
-    is(iterator.nextNode().nodeName, "span",
-       "iterator.nextNode() returned the wrong node");
+    is(iter.nextNode().nodeName, "div",
+       "iter.nextNode() returned the wrong node");
+    is(iter.nextNode().nodeName, "span",
+       "iter.nextNode() returned the wrong node");
     try {
-      var p = iterator.nextNode();
-      ok(false, "iterator.nextNode() should have thrown, but instead it returned <" + p.nodeName + ">");
+      var p = iter.nextNode();
+      ok(false, "iter.nextNode() should have thrown, but instead it returned <" + p.nodeName + ">");
     } catch (x) { ok(true, x) }
   })();
-        
+
 ]]></script>
 </pre>
 </body>
diff --git a/dom/base/test/test_bug1037687_subframe.html b/dom/base/test/test_bug1037687_subframe.html
index 141cddaa5a25..4258f772b677 100644
--- a/dom/base/test/test_bug1037687_subframe.html
+++ b/dom/base/test/test_bug1037687_subframe.html
@@ -7,7 +7,6 @@
   var is = window.parent.is;
 
   var host;
-  var sr;
   var embed;
   var object;
   var iframe;
@@ -34,7 +33,7 @@
 
   function test() {
     host = document.getElementById("host");
-    sr = host.attachShadow({mode: 'open'});
+    let sr = host.attachShadow({mode: 'open'});
     embed = createResource(sr, "embed");
     object = createResource(sr, "object");
     iframe = createResource(sr, "iframe");
diff --git a/dom/base/test/test_bug1091883.html b/dom/base/test/test_bug1091883.html
index a3d5f2d4fc4a..0b38c59d43ba 100644
--- a/dom/base/test/test_bug1091883.html
+++ b/dom/base/test/test_bug1091883.html
@@ -43,7 +43,7 @@ function getTargetOrigin(i) {
 }
 
 // Create the frames, and tell them which subframes to load.
-for (var i = 0; i < numTests; i++) {
+for (let i = 0; i < numTests; i++) {
   var frame = document.createElement("iframe");
   frame.src = getFrameOrigin(i) +
               "/tests/dom/base/test/file_bug1091883_frame.html#" +
@@ -53,7 +53,7 @@ for (var i = 0; i < numTests; i++) {
 
 // Navigate all subframes to the target.
 window.onload = function() {
-  for (var i = 0; i < numTests; i++) {
+  for (let i = 0; i < numTests; i++) {
     frames[i].frames[0].location = getTargetOrigin(i) +
         "/tests/dom/base/test/file_bug1091883_target.html#" + i;
   }
diff --git a/dom/base/test/test_bug338583.html b/dom/base/test/test_bug338583.html
index 18ea2194ef39..87cd83561017 100644
--- a/dom/base/test/test_bug338583.html
+++ b/dom/base/test/test_bug338583.html
@@ -58,14 +58,14 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=338583
       doTest5, doTest5_b, doTest5_c, doTest5_e, doTest6, doTest7
     ];
 
-    for (var test_id=0; test_id < all_tests.length; ++test_id) {
+    for (let test_id=0; test_id < all_tests.length; ++test_id) {
       gTestsHaveFinished[test_id] = false;
       var fn = all_tests[test_id];
       fn(test_id);
     }
 
     setTimeout(function() {
-      for (var test_id=0; test_id < all_tests.length; ++test_id) {
+      for (let test_id=0; test_id < all_tests.length; ++test_id) {
         if (!gTestsHaveFinished[test_id]) {
           ok(false, "Test " + test_id + " took too long");
           setTestHasFinished(test_id);
@@ -181,7 +181,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=338583
 
   function doTest1_f(test_id) {
     var called_on_error = false;
-  
+
     gEventSourceObj1_f = new EventSource("file_bug869432.eventsource");
     gEventSourceObj1_f.onopen = function(e) {
       ok(false, "Test 1.f failed: onopen was called");
@@ -429,8 +429,8 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=338583
 //   b) invalid access-control xsite request
 //   c) valid access-control xsite request on a restricted page with credentials
 //   d) valid access-control xsite request on a restricted page without credentials
-//   e) valid access-control xsite request on a restricted page when the parameter withCredentials is a getter 
-//   f) valid access-control xsite request on a restricted page when the parameter withCredentials is missing 
+//   e) valid access-control xsite request on a restricted page when the parameter withCredentials is a getter
+//   f) valid access-control xsite request on a restricted page when the parameter withCredentials is missing
 
   function doTest5(test_id)
   {
@@ -498,17 +498,17 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=338583
     xhr.send();
     xhr.onloadend = function() {
       ok(xhr.status == 200, "Failed to set credentials in test 5.d");
-  
+
       gEventSourceObj5_d = new EventSource("https://example.com/tests/dom/base/test/file_restrictedEventSource.sjs?test=user2_evtsrc");
       ok(!gEventSourceObj5_d.withCredentials, "Wrong withCredentials in test 5.d");
-  
+
       gEventSourceObj5_d.onmessage = function(e) {
         ok(e.origin == "https://example.com", "Wrong Origin in test 5.d");
         fn_onmessage(e);
       };
       gEventSourceObj5_d.hits = [];
       gEventSourceObj5_d.hits['fn_onmessage'] = 0;
-  
+
       setTimeout(function() {
         ok(gEventSourceObj5_d.hits['fn_onmessage'] == 0, "Test 5.d failed");
         gEventSourceObj5_d.close();
@@ -605,10 +605,10 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=338583
     {
       e.target.msg_received.push(e.data);
     }
-    
+
     setTimeout(function() {
       gEventSourceObj7.close();
-      
+
       ok(gEventSourceObj7.msg_received[0] == "" &&
          gEventSourceObj7.msg_received[1] == "delayed1" &&
          gEventSourceObj7.msg_received[2] == "delayed2", "Test 7 failed");
@@ -663,4 +663,3 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=338583
   <span id=waitSpan>Wait please...</span>
 </body>
 </html>
-
diff --git a/dom/base/test/test_bug409380.html b/dom/base/test/test_bug409380.html
index 5917059efe86..ddc796b22375 100644
--- a/dom/base/test/test_bug409380.html
+++ b/dom/base/test/test_bug409380.html
@@ -12,13 +12,14 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=409380
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=409380">Mozilla Bug 409380</a>
 <p id="display"></p>
 <div id="content" style="display: none">
-  
+
 </div>
 <pre id="test">
 <script class="testbody" type="text/javascript">
 
 /** Test for Bug 409380 **/
 
+// eslint-disable-next-line complexity
 function runRangeTest()
 {
   // Bug 336381
@@ -363,7 +364,7 @@ function runRangeTest()
     hadException = true;
   }
   ok(!hadException, "Shouldn't have got an exception!");
-  
+
   SimpleTest.finish();
 }
 
@@ -375,4 +376,3 @@ addLoadEvent(runRangeTest);
 <div id="connectedDiv">zz</div>
 </body>
 </html>
-
diff --git a/dom/base/test/test_bug431701.html b/dom/base/test/test_bug431701.html
index 0d30853377bd..4009fc017af7 100644
--- a/dom/base/test/test_bug431701.html
+++ b/dom/base/test/test_bug431701.html
@@ -37,7 +37,7 @@ var docSources = [
   "iframe7_bug431701.xml",
 ];
 
-for (var i = 0; i < docSources.length; ++i) {
+for (let i = 0; i < docSources.length; ++i) {
   document.getElementsByTagName("iframe")[i].src = docSources[i];
 }
 
@@ -94,7 +94,7 @@ function startTest() {
   // sanity check
   isnot("", null, "Shouldn't be equal!");
 
-  for (var i = 0; i < tests.length; ++i) {
+  for (let i = 0; i < tests.length; ++i) {
     doTest(i);
   }
 
@@ -106,7 +106,7 @@ function startTest() {
                 .getRequestHeader("Content-Type"),
      "application/xml;charset=UTF-8", "Testing correct type on the wire");
   xhr.abort();
-                     
+
   SimpleTest.finish();
 };
 
@@ -117,4 +117,3 @@ function startTest() {
 </pre>
 </body>
 </html>
-
diff --git a/dom/base/test/test_bug435425.html b/dom/base/test/test_bug435425.html
index 7be338220fe3..a609a0bac94c 100644
--- a/dom/base/test/test_bug435425.html
+++ b/dom/base/test/test_bug435425.html
@@ -12,14 +12,13 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=435425
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=435425">Mozilla Bug 435425</a>
 <p id="display"></p>
 <div id="content" style="display: none">
-  
+
 </div>
 <pre id="test">
 <script class="testbody" type="text/javascript">
 
 /** Test for Bug 435425 **/
 
-var xhr = null;
 var upload = null;
 var currentEvents = null;
 var expectedResponseText = null;
@@ -69,7 +68,7 @@ function logEvent(evt) {
   ok(evt.target instanceof currentEvents[i].target,
      "Wrong event target [" + evt.target + "," + evt.type + "]!");
 
-  // If we handled non-optional event, remove all optional events before the 
+  // If we handled non-optional event, remove all optional events before the
   // handled event and then the non-optional event from the list.
   if (!currentEvents[i].optional) {
     for (;i != -1; --i) {
@@ -102,7 +101,7 @@ function openXHR(xhr, method, url, privileged) {
 }
 
 function start(obj) {
-  xhr = new XMLHttpRequest();
+  let xhr = new XMLHttpRequest();
   upload = xhr.upload;
   currentEvents = obj.expectedEvents;
   expectedResponseText = obj.withUpload;
@@ -208,7 +207,7 @@ for (var largeLength = 0; largeLength < (0x0000FFFF + 2); ++largeLength) {
 const XHR = XMLHttpRequest;
 const UPLOAD = XMLHttpRequestUpload;
 
-var tests = 
+var tests =
   [
     { method: "GET", withUpload: none, testAbort: false, testRedirectError: false, testNetworkError: false,
       expectedEvents: [{target: XHR, type: "loadstart", optional: false},
@@ -429,4 +428,3 @@ addLoadEvent(runTest);
 </pre>
 </body>
 </html>
-
diff --git a/dom/base/test/test_bug444546.html b/dom/base/test/test_bug444546.html
index 2769d77b6fa4..4fec1754e03b 100644
--- a/dom/base/test/test_bug444546.html
+++ b/dom/base/test/test_bug444546.html
@@ -29,7 +29,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=444546
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=444546">Mozilla Bug 444546</a>
 <p id="display"></p>
 <div id="content" style="display: none">
-  
+
 </div>
 <pre id="test">
 <script type="application/javascript">
@@ -60,9 +60,9 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=444546
     evt.target.log.textContent = diff + "ms";
     evt.target.prevTime = time;
     if (evt.lengthComputable) {
-      var loaded = (evt.loaded / evt.total);
-      if (loaded < 1) {
-        evt.target.log.style.width = (loaded * 400) + "px";
+      var fractionLoaded = (evt.loaded / evt.total);
+      if (fractionLoaded < 1) {
+        evt.target.log.style.width = (fractionLoaded * 400) + "px";
       }
     }
   }
diff --git a/dom/base/test/test_bug482935.html b/dom/base/test/test_bug482935.html
index a7c2230833f5..444c3aada781 100644
--- a/dom/base/test/test_bug482935.html
+++ b/dom/base/test/test_bug482935.html
@@ -26,7 +26,7 @@ function testCancelInPhase4() {
 
   // First request - should be loaded from server
   var xhr = new XMLHttpRequest();
-  xhr.addEventListener("readystatechange", function(e) {
+  xhr.addEventListener("readystatechange", function() {
     if (xhr.readyState < xhr.DONE) return;
     is(xhr.readyState, xhr.DONE, "wrong readyState");
     xhr.abort();
diff --git a/dom/base/test/test_bug493881.js b/dom/base/test/test_bug493881.js
index 0ae31eb20408..575339db083f 100644
--- a/dom/base/test/test_bug493881.js
+++ b/dom/base/test/test_bug493881.js
@@ -1,6 +1,6 @@
-/** 
+/**
  * Test for Bug 493881: Changes to legacy HTML color properties before the BODY is loaded
- * should be ignored. Additionally, after BODY loads, setting any of these properties to undefined 
+ * should be ignored. Additionally, after BODY loads, setting any of these properties to undefined
  * should cause them to be returned as the string "undefined".
  */
 
@@ -26,7 +26,7 @@ function setAndTestProperty(prop, color) {
  * Attempt to set legacy color properties before BODY exists, and verify that such
  * attempts are ignored.
  */
-for (var i = 0; i < legacyProps.length; i++) {
+for (let i = 0; i < legacyProps.length; i++) {
   initialValues[i] = setAndTestProperty(legacyProps[i], testColors[i]);
 }
 
@@ -35,13 +35,13 @@ for (var i = 0; i < legacyProps.length; i++) {
  */
 addLoadEvent( function() {
   // Verify that the legacy color properties still have their original values.
-  for (var i = 0; i < legacyProps.length; i++) {
+  for (let i = 0; i < legacyProps.length; i++) {
     is(document[legacyProps[i]], initialValues[i], "document[" + legacyProps[i] + "] altered after body load");
   }
-  
+
   // Verify that legacy color properties applied before BODY are really ignored when rendering.
   // Save current computed style colors for later use.
-  for (i = 0; i < idPropList.length; i++) {
+  for (let i = 0; i < idPropList.length; i++) {
     var style = window.getComputedStyle(document.getElementById(idPropList[i].id));
     var color = style.getPropertyValue(idPropList[i].prop);
     idPropList[i].initialComputedColor = color;
@@ -49,21 +49,21 @@ addLoadEvent( function() {
   }
   // XXX: Can't get links to visually activate via script events, so can't verify
   // that the alinkColor property was not applied.
-  
+
   // Verify that setting legacy color props to undefined after BODY loads will cause them
   // to be read as the string "undefined".
-  for (var i = 0; i < legacyProps.length; i++) {
+  for (let i = 0; i < legacyProps.length; i++) {
     document[legacyProps[i]] = undefined;
-    is(document[legacyProps[i]], "undefined", 
+    is(document[legacyProps[i]], "undefined",
       "Unexpected value of " + legacyProps[i] + " after setting to undefined");
   }
-  
+
   // Verify that setting legacy color props to undefined led to result
   // of parsing undefined as a color.
-  for (i = 0; i < idPropList.length; i++) {
+  for (let i = 0; i < idPropList.length; i++) {
     var style = window.getComputedStyle(document.getElementById(idPropList[i].id));
     var color = style.getPropertyValue(idPropList[i].prop);
-    is(color, "rgb(0, 239, 14)", 
+    is(color, "rgb(0, 239, 14)",
       "element's style should get result of parsing undefined as a color");
   }
 
diff --git a/dom/base/test/test_bug682463.html b/dom/base/test/test_bug682463.html
index 605315ab6b75..91b06b74077a 100644
--- a/dom/base/test/test_bug682463.html
+++ b/dom/base/test/test_bug682463.html
@@ -12,7 +12,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=682463
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=682463">Mozilla Bug 682463</a>
 <p id="display"></p>
 <div id="content" style="display: none">
-  
+
 </div>
 <pre id="test">
 <script type="application/javascript">
@@ -23,10 +23,10 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=682463
     return document.createTextNode(s);
   }
   function div(l,r) {
-    var div = document.createElement("DIV");
-    if (l) div.appendChild(l);
-    if (r) div.appendChild(r);
-    return div;
+    var d = document.createElement("DIV");
+    if (l) d.appendChild(l);
+    if (r) d.appendChild(r);
+    return d;
   }
   function createRange(sn,so,en,eo) {
     var r = document.createRange();
diff --git a/dom/base/test/test_bug704320-1.html b/dom/base/test/test_bug704320-1.html
index 3a3029d3f773..d20e6c0e1da6 100644
--- a/dom/base/test/test_bug704320-1.html
+++ b/dom/base/test/test_bug704320-1.html
@@ -53,18 +53,18 @@ SimpleTest.requestLongerTimeout(4);
  * This is the main test routine -- serialized by use of a generator.
  * It performs all tests in sequence using four iframes.
  */
-function startTests(testName, testIframeUrls) {
+function startTests(testName, iframeUrls) {
   testsGenerator[testName] = (function*() {
     var testframe = document.getElementById(testName);
     testframe.onload = function() {
       advance(testName);
     }
 
-    // load the test frame from testIframeUrls[url]
+    // load the test frame from iframeUrls[url]
     // it will call back into this function via postMessage when it finishes
     // loading and continue beyond the yield.
-    for(url in testIframeUrls) {
-      yield testframe.src = testIframeUrls[url];
+    for(url in iframeUrls) {
+      yield testframe.src = iframeUrls[url];
       // run test and check result for loaded test URL
       yield checkExpectedGlobalResults(testName);
     }
diff --git a/dom/base/test/test_bug704320-2.html b/dom/base/test/test_bug704320-2.html
index 6564fc33d720..3c02e1d8d7bc 100644
--- a/dom/base/test/test_bug704320-2.html
+++ b/dom/base/test/test_bug704320-2.html
@@ -53,18 +53,18 @@ SimpleTest.requestLongerTimeout(4);
  * This is the main test routine -- serialized by use of a generator.
  * It performs all tests in sequence using four iframes.
  */
-function startTests(testName, testIframeUrls) {
+function startTests(testName, iframeUrls) {
   testsGenerator[testName] = (function*() {
     var testframe = document.getElementById(testName);
     testframe.onload = function() {
       advance(testName);
     }
 
-    // load the test frame from testIframeUrls[url]
+    // load the test frame from iframeUrls[url]
     // it will call back into this function via postMessage when it finishes
     // loading and continue beyond the yield.
-    for(url in testIframeUrls) {
-      yield testframe.src = testIframeUrls[url];
+    for(url in iframeUrls) {
+      yield testframe.src = iframeUrls[url];
       // run test and check result for loaded test URL
       yield checkExpectedGlobalResults(testName);
     }
diff --git a/dom/base/test/test_bug782342.html b/dom/base/test/test_bug782342.html
index a261d5f4141a..38ad2758e8c0 100644
--- a/dom/base/test/test_bug782342.html
+++ b/dom/base/test/test_bug782342.html
@@ -25,8 +25,8 @@ var xhr = new XMLHttpRequest();
 xhr.open("GET", url, true);
 xhr.responseType = "blob";
 
-function checkHeaders(xhr) {
-  var headers = xhr.getAllResponseHeaders();
+function checkHeaders(xhrInner) {
+  var headers = xhrInner.getAllResponseHeaders();
   dump(headers + "\n");
   var p = headers.split("\n");
 
@@ -44,9 +44,9 @@ function checkHeaders(xhr) {
   ok(contentLength == true, "Content-length is part of the headers!");
   ok(contentType == true, "Content-type is part of the headers!");
 
-  var ct = xhr.getResponseHeader('content-type');
+  var ct = xhrInner.getResponseHeader('content-type');
   ok(ct.length > 0, 'Get Response Header - content type: ' + ct);
-  var cl = xhr.getResponseHeader('content-length');
+  var cl = xhrInner.getResponseHeader('content-length');
   ok(cl.length > 0, 'Get Response Header - content length: ' + cl);
 }
 
diff --git a/dom/base/test/test_bug913761.html b/dom/base/test/test_bug913761.html
index 085630e55a1f..6f08e5a1d2e3 100644
--- a/dom/base/test/test_bug913761.html
+++ b/dom/base/test/test_bug913761.html
@@ -18,7 +18,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=913761
   transportChannel.port1.onmessage = function (event) {
     ok(true, 'Port Returned.');
     var portToService = event.data.port;
-    portToService.onmessage = function (event) {
+    portToService.onmessage = function () {
       ok(true, "message received");
       SimpleTest.finish();
     };
diff --git a/dom/base/test/test_bug922681.html b/dom/base/test/test_bug922681.html
index dab41f4bb998..16b91f70be5c 100644
--- a/dom/base/test/test_bug922681.html
+++ b/dom/base/test/test_bug922681.html
@@ -76,11 +76,11 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=922681
     testInnerHTML(c, "<body>", "");
 
     function testTableBody(tag) {
-        var c = document.createElement(tag);
-        testInnerHTML(c, "abc", "abc");
-        testInnerHTML(c, "<td>", "<tr><td></td></tr>");
-        testInnerHTML(c, "</body>", "");
-        testInnerHTML(c, "<input>", "<input>");
+        var e = document.createElement(tag);
+        testInnerHTML(e, "abc", "abc");
+        testInnerHTML(e, "<td>", "<tr><td></td></tr>");
+        testInnerHTML(e, "</body>", "");
+        testInnerHTML(e, "<input>", "<input>");
     }
     testTableBody("thead");
     testTableBody("tbody");
diff --git a/dom/base/test/test_bug976673.html b/dom/base/test/test_bug976673.html
index d12935999761..ff656ef22117 100644
--- a/dom/base/test/test_bug976673.html
+++ b/dom/base/test/test_bug976673.html
@@ -52,23 +52,23 @@ function testSetFocus(aEventType, aCallback)
 
     var movingFocus = false;
     button.addEventListener(aEventType,
-                            function (aEvent) {
+                            function (event) {
                               movingFocus = true;
                               input.focus();
-                              aEvent.preventDefault();
+                              event.preventDefault();
                               button.removeEventListener(aEventType, arguments.callee, true);
                             }, true);
 
     synthesizeMouseAtCenter(button, {});
 
-    window.addEventListener("message", function (aEvent) {
+    window.addEventListener("message", function (event) {
       if (movingFocus) {
-        is(aEvent.data, "input-value: blur",
+        is(event.data, "input-value: blur",
            description + "<input> in the iframe should get blur");
         is(input.value, "focus",
            description + "<input> in the parent should get focus");
       } else {
-        is(aEvent.data, "input-value: focus",
+        is(event.data, "input-value: focus",
            description + "<input> in the iframe should keep having focus");
       }
 
diff --git a/dom/base/test/test_intersectionobservers.html b/dom/base/test/test_intersectionobservers.html
index 9540c86d5aed..456874498234 100644
--- a/dom/base/test/test_intersectionobservers.html
+++ b/dom/base/test/test_intersectionobservers.html
@@ -29,6 +29,7 @@ limitations under the License.
 <p id="display"></p>
 <pre id="test">
 <script type="application/javascript">
+  /* eslint "no-shadow": ["error", {"allow": ["done", "next"]}] */
   var tests = [];
   var curDescribeMsg = '';
   var curItMsg = '';
@@ -149,18 +150,18 @@ limitations under the License.
 
   var sinon = {
     spy: function () {
-      var callbacks = [];
+      var cbs = [];
       var fn = function () {
         fn.callCount++;
         fn.lastCall = { args: arguments };
-        if (callbacks.length) {
-          callbacks.shift()();
+        if (cbs.length) {
+          cbs.shift()();
         }
       };
       fn.callCount = 0;
       fn.lastCall = { args: [] };
-      fn.waitForNotification = (fn) => {
-        callbacks.push(fn);
+      fn.waitForNotification = (fn1) => {
+        cbs.push(fn1);
       };
       return fn;
     }
@@ -927,7 +928,7 @@ limitations under the License.
     });
 
     describe('observe subframe', function () {
-      
+
       it('should not trigger if target and root are not in the same document',
           function(done) {
 
@@ -944,7 +945,7 @@ limitations under the License.
         }
 
         targetEl4.src = "intersectionobserver_iframe.html";
-      
+
       });
 
       it('boundingClientRect matches target.getBoundingClientRect() for an element inside an iframe',
@@ -977,7 +978,7 @@ limitations under the License.
         targetEl4.src = "http://example.org/tests/dom/base/test/intersectionobserver_cross_domain_iframe.html";
 
       });
-    
+
     });
 
     describe('takeRecords', function() {
@@ -1002,7 +1003,7 @@ limitations under the License.
             done();
           });
         });
-        
+
       });
 
     });
diff --git a/dom/base/test/test_messagePort.html b/dom/base/test/test_messagePort.html
index d1e1db73b1e1..c4fc9d11ce41 100644
--- a/dom/base/test/test_messagePort.html
+++ b/dom/base/test/test_messagePort.html
@@ -21,24 +21,24 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=912456
     function receiveMessage(evt) {
       ok(evt.data.port, "Port has been received!");
 
-      var a = new MessageChannel();
-      ok(a, "MessageChannel created");
+      var a1 = new MessageChannel();
+      ok(a1, "MessageChannel created");
 
       try {
-        evt.data.port.postMessage({port: a.port2});
+        evt.data.port.postMessage({port: a1.port2});
         ok(false, "PostMessage should throw! - no transfered port");
       } catch(e) {
         ok(true, "PostMessage should throw! - no transfered port");
       }
 
       try {
-        evt.data.port.postMessage({port: a.port2}, [a.port2, a.port2]);
+        evt.data.port.postMessage({port: a1.port2}, [a1.port2, a1.port2]);
         ok(false, "PostMessage should throw - no duplicate!");
       } catch(e) {
         ok(true, "PostMessage should throw - no duplicate!");
       }
 
-      evt.data.port.postMessage({port: a.port2}, [a.port2]);
+      evt.data.port.postMessage({port: a1.port2}, [a1.port2]);
     }
 
     a.port1.onmessage = function(evt) {
@@ -74,10 +74,10 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=912456
       ok(evt.ports, "Port is received");
       is(evt.ports.length, 1, "Ports.length is 1");
 
-      var a = new MessageChannel();
-      ok(a, "MessageChannel created");
+      var a1 = new MessageChannel();
+      ok(a1, "MessageChannel created");
 
-      evt.ports[0].postMessage(42, [a.port2]);
+      evt.ports[0].postMessage(42, [a1.port2]);
     }
 
     a.port1.onmessage = function(evt) {
@@ -112,4 +112,3 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=912456
 </script>
 </body>
 </html>
-
diff --git a/dom/base/test/test_messagemanager_targetchain.html b/dom/base/test/test_messagemanager_targetchain.html
index f7e056b7d728..0be6fb1f7be8 100644
--- a/dom/base/test/test_messagemanager_targetchain.html
+++ b/dom/base/test/test_messagemanager_targetchain.html
@@ -75,7 +75,7 @@
             info("Third iframe loaded");
             let doc3 = SpecialPowers.wrap(iframe3).contentDocument;
             let target = doc3.getElementById("target");
-            target.addEventListener("test-event", function onEvent(e) {
+            target.addEventListener("test-event", function onEvent() {
               is(messageIndex, 2,
                  "target should be the last one to receive the test event");
               messageIndex++;
diff --git a/dom/base/test/test_mutationobservers.html b/dom/base/test/test_mutationobservers.html
index c768708900ba..450f55ab7ad9 100644
--- a/dom/base/test/test_mutationobservers.html
+++ b/dom/base/test/test_mutationobservers.html
@@ -9,11 +9,11 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=641821
   <script src="/tests/SimpleTest/SimpleTest.js"></script>
   <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
 </head>
-<body onload="runTest()">                                                  
+<body onload="runTest()">
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=641821">Mozilla Bug 641821</a>
 <p id="display"></p>
 <div id="content" style="display: none">
-                                
+
 </div>
 <pre id="test">
 <script type="application/javascript">
@@ -53,14 +53,14 @@ if (!("ok" in window)) {
 
 if (!("is" in window)) {
   window.is = function(val, refVal, str) {
-    log(str + (val == refVal? " PASSED " : " FAILED ") + 
+    log(str + (val == refVal? " PASSED " : " FAILED ") +
         (val != refVal ? "expected " + refVal + " got " + val + "\n" : "\n"));
   }
 }
 
 if (!("isnot" in window)) {
   window.isnot = function(val, refVal, str) {
-    log(str + (val != refVal? " PASSED " : " FAILED ") + 
+    log(str + (val != refVal? " PASSED " : " FAILED ") +
         (val == refVal ? "Didn't expect " + refVal + "\n" : "\n"));
   }
 }
@@ -367,10 +367,10 @@ function testNestedMutations() {
     is(records[0].type, "characterData", "Should have got characterData");
     observer.disconnect();
     m = null;
-    m3 = new M(function(records, observer) {
+    m3 = new M(function(recordsInner, observerInnder) {
       ok(m2WasCalled, "m2 should have been called before m3!");
-      is(records[0].type, "characterData", "Should have got characterData");
-      observer.disconnect();
+      is(recordsInner[0].type, "characterData", "Should have got characterData");
+      observerInnder.disconnect();
       then(testAdoptNode);
       m3 = null;
     });
@@ -464,7 +464,7 @@ function testInsertAdjacentHTML() {
       is(records[2].nextSibling, null, "");
       is(records[3].target, doc.body, "");
       is(records[3].previousSibling, d4, "");
-      is(records[3].nextSibling, null, "");      
+      is(records[3].nextSibling, null, "");
       observer.disconnect();
       then(testSyncXHR);
       m = null;
@@ -518,12 +518,12 @@ function testTakeRecords() {
       is(records[1].addedNodes.length, 2, "Should have got addedNodes");
       is(records[2].type, "attributes", "Should have got attributes");
       is(records[2].attributeName, "foo", "");
-      
+
       is(records.length, takenRecords.length, "Should have had similar mutations");
       is(records[0].type, takenRecords[0].type, "Should have had similar mutations");
       is(records[1].type, takenRecords[1].type, "Should have had similar mutations");
       is(records[2].type, takenRecords[2].type, "Should have had similar mutations");
-      
+
       is(records[1].removedNodes.length, takenRecords[1].removedNodes.length, "Should have had similar mutations");
       is(records[1].addedNodes.length, takenRecords[1].addedNodes.length, "Should have had similar mutations");
 
@@ -570,7 +570,7 @@ function testTakeRecords() {
 }
 
 function testExpandos() {
-  var m2 = new M(function(records, observer) {
+  m2 = new M(function(records, observer) {
     is(observer.expandoProperty, true);
     observer.disconnect();
     then(testOutsideShadowDOM);
@@ -596,7 +596,7 @@ function testOutsideShadowDOM() {
     then(testMarquee);
     return;
   }
-  var m = new M(function(records, observer) {
+  m = new M(function(records, observer) {
     is(records.length, 1);
     is(records[0].type, "attributes", "Should have got attributes");
     observer.disconnect();
@@ -617,7 +617,7 @@ function testOutsideShadowDOM() {
 }
 
 function testMarquee() {
-  var m = new M(function(records, observer) {
+  m = new M(function(records, observer) {
     is(records.length, 1);
     is(records[0].type, "attributes");
     is(records[0].attributeName, "ok");
@@ -724,7 +724,7 @@ function testStyleRemoveProperty2() {
 
 function testAttributeRecordMerging1() {
   ok(true, "testAttributeRecordMerging1");
-  var m = new M(function(records, observer) {
+  m = new M(function(records, observer) {
     is(records.length, 2);
     is(records[0].type, "attributes");
     is(records[0].target, div);
@@ -757,7 +757,7 @@ function testAttributeRecordMerging1() {
 
 function testAttributeRecordMerging2() {
   ok(true, "testAttributeRecordMerging2");
-  var m = new M(function(records, observer) {
+  m = new M(function(records, observer) {
     is(records.length, 2);
     is(records[0].type, "attributes");
     is(records[0].target, div);
@@ -794,7 +794,7 @@ function testAttributeRecordMerging2() {
 
 function testAttributeRecordMerging3() {
   ok(true, "testAttributeRecordMerging3");
-  var m = new M(function(records, observer) {
+  m = new M(function(records, observer) {
     is(records.length, 4);
     is(records[0].type, "attributes");
     is(records[0].target, div);
@@ -807,7 +807,7 @@ function testAttributeRecordMerging3() {
     is(records[1].attributeName, "foo");
     is(records[1].attributeNamespace, null);
     is(records[1].oldValue, "initial");
-    
+
     is(records[2].type, "attributes");
     is(records[2].target, div);
     is(records[2].attributeName, "foo");
@@ -845,7 +845,7 @@ function testAttributeRecordMerging3() {
 
 function testAttributeRecordMerging4() {
   ok(true, "testAttributeRecordMerging4");
-  var m = new M(function(records, observer) {
+  m = new M(function(records, observer) {
   });
 
   div.setAttribute("foo", "initial");
diff --git a/dom/base/test/test_navigator_language.html b/dom/base/test/test_navigator_language.html
index ced6a2ba88af..7d8cca5fc851 100644
--- a/dom/base/test/test_navigator_language.html
+++ b/dom/base/test/test_navigator_language.html
@@ -119,8 +119,8 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=889335
     function* testEvents() {
       for (var i = 0; i < testValues.length; ++i) {
         var data = testValues[i];
-        setTimeout(function(data) {
-          SpecialPowers.pushPrefEnv({"set": [['intl.accept_languages', data.accept_languages]]});
+        setTimeout(function(d) {
+          SpecialPowers.pushPrefEnv({"set": [['intl.accept_languages', d.accept_languages]]});
         }, 0, data);
         expectedLanguageChanges++;
         yield undefined;
diff --git a/dom/base/test/test_postMessages.html b/dom/base/test/test_postMessages.html
index 34f88612f955..f47a7ea63ebd 100644
--- a/dom/base/test/test_postMessages.html
+++ b/dom/base/test/test_postMessages.html
@@ -403,7 +403,7 @@ function test_workers() {
   w.onmessage = function(e) {
     is(e.data, 'ok', "Worker ready!");
 
-    w.onmessage = function(e) {
+    w.onmessage = function(e1) {
       if (!resolve) {
         ok(false, "Unexpected message!");
         return;
@@ -411,7 +411,7 @@ function test_workers() {
 
       let tmp = resolve;
       resolve = null;
-      tmp({ data: e.data, ports: e.ports });
+      tmp({ data: e1.data, ports: e1.ports });
     }
 
     runTests({
@@ -423,7 +423,7 @@ function test_workers() {
           resolve = r;
           try {
             w.postMessage(what, ports);
-          } catch(e) {
+          } catch(ex) {
             resolve = null;
             rr();
           }
@@ -493,7 +493,7 @@ function test_broadcastChannel_inWorkers() {
   w.onmessage = function(e) {
     is(e.data, 'ok', "Worker ready!");
 
-    w.onmessage = function(e) {
+    w.onmessage = function(e1) {
       if (!resolve) {
         ok(false, "Unexpected message!");
         return;
@@ -501,7 +501,7 @@ function test_broadcastChannel_inWorkers() {
 
       let tmp = resolve;
       resolve = null;
-      tmp({ data: e.data, ports: e.ports });
+      tmp({ data: e1.data, ports: e1.ports });
     }
 
     runTests({
@@ -581,7 +581,7 @@ function test_messagePort_inWorkers() {
   w.onmessage = function(e) {
     is(e.data, 'ok', "Worker ready!");
 
-    w.onmessage = function(e) {
+    w.onmessage = function(e1) {
       if (!resolve) {
         ok(false, "Unexpected message!");
         return;
@@ -589,7 +589,7 @@ function test_messagePort_inWorkers() {
 
       let tmp = resolve;
       resolve = null;
-      tmp({ data: e.data, ports: e.ports });
+      tmp({ data: e1.data, ports: e1.ports });
     }
 
     runTests({
@@ -601,7 +601,7 @@ function test_messagePort_inWorkers() {
           resolve = r;
           try {
             mc.port1.postMessage(what, ports);
-          } catch(e) {
+          } catch(ex) {
             resolve = null;
             rr();
           }
diff --git a/dom/base/test/test_range_bounds.html b/dom/base/test/test_range_bounds.html
index 32a04842ffa3..657d3151987d 100644
--- a/dom/base/test/test_range_bounds.html
+++ b/dom/base/test/test_range_bounds.html
@@ -90,8 +90,8 @@ function runATest(obj) {
         //convert RectList to a real array
         obj.rectList=Array.prototype.slice.call(obj.rectList, 0);
       }
-      obj.rectList.forEach(function(rect,i) {
-        is(_getRect(rectlist[i]),_getRect(rect),
+      obj.rectList.forEach(function(r,i) {
+        is(_getRect(rectlist[i]),_getRect(r),
            annotateName(testname+": item at "+i));
       });
     }
diff --git a/dom/base/test/test_timeout_clamp.html b/dom/base/test/test_timeout_clamp.html
index a260b6704afb..bde8ba7e6266 100644
--- a/dom/base/test/test_timeout_clamp.html
+++ b/dom/base/test/test_timeout_clamp.html
@@ -20,10 +20,9 @@ SimpleTest.waitForExplicitFinish();
 // nesting.
 function clearNestingLevel() {
   return new Promise(resolve => {
-    window.addEventListener('message', function onMessage() {
-      window.removeEventListener('message', onMessage);
+    window.addEventListener('message', () => {
       resolve();
-    });
+    }, {once: true});
     postMessage('done', '*');
   });
 }