diff --git a/.eslintignore b/.eslintignore
index 5819f278fdf9..114812d3b7cd 100644
--- a/.eslintignore
+++ b/.eslintignore
@@ -138,12 +138,17 @@ intl/l10n/
 # Exclude everything but self-hosted JS
 js/examples/
 js/public/
-js/xpconnect/
 js/src/devtools/
 js/src/jit-test/
 js/src/tests/
 js/src/Y.js
 
+# Changes to XPConnect tests must be carefully audited.
+js/xpconnect/loader/
+js/xpconnect/tests/browser/
+js/xpconnect/tests/mochitest/
+js/xpconnect/tests/unit/
+
 # Fuzzing code for testing only, targeting the JS shell
 js/src/fuzz-tests/
 
diff --git a/js/xpconnect/tests/chrome/bug503926.xhtml b/js/xpconnect/tests/chrome/bug503926.xhtml
index 07f4ec236aac..bfd5470305d6 100644
--- a/js/xpconnect/tests/chrome/bug503926.xhtml
+++ b/js/xpconnect/tests/chrome/bug503926.xhtml
@@ -18,7 +18,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=503926
       var gWindowUtils = window.windowUtils;
       var passed = false;
       // eslint-disable-next-line mozilla/use-chromeutils-generateqi
-      var obj = { QueryInterface: function() { passed = true; } }
+      var obj = { QueryInterface() { passed = true; } }
       gWindowUtils.xpconnectArgument(obj);
       var isDialog = location.hash != '#iframe';
       var outer = XPCNativeWrapper.unwrap(isDialog ? window.arguments[0] : parent);
diff --git a/js/xpconnect/tests/chrome/file_expandosharing.jsm b/js/xpconnect/tests/chrome/file_expandosharing.jsm
index 78718b28ec1c..f680ae20a993 100644
--- a/js/xpconnect/tests/chrome/file_expandosharing.jsm
+++ b/js/xpconnect/tests/chrome/file_expandosharing.jsm
@@ -1,4 +1,4 @@
-var EXPORTED_SYMBOLS = ['checkFromJSM'];
+var EXPORTED_SYMBOLS = ["checkFromJSM"];
 
 function checkFromJSM(target, is_op) {
   is_op(target.numProp, 42, "Number expando works");
@@ -6,5 +6,7 @@ function checkFromJSM(target, is_op) {
   // If is_op is todo_is, target.objProp will be undefined.
   try {
     is_op(target.objProp.bar, "baz", "Object expando works");
-  } catch(e) { is_op(0, 1, "No object expando"); }
-}
\ No newline at end of file
+  } catch (e) {
+    is_op(0, 1, "No object expando");
+  }
+}
diff --git a/js/xpconnect/tests/chrome/outoflinexulscript.js b/js/xpconnect/tests/chrome/outoflinexulscript.js
index a06b633afd99..14b99048e9c9 100644
--- a/js/xpconnect/tests/chrome/outoflinexulscript.js
+++ b/js/xpconnect/tests/chrome/outoflinexulscript.js
@@ -1,6 +1,5 @@
 // Some unicode characters that must be decoded:
 // ………………………………………………………………………………………………………………………………
 function outoflinefunction() {
-    return 42;
+  return 42;
 }
-
diff --git a/js/xpconnect/tests/chrome/test_APIExposer.xhtml b/js/xpconnect/tests/chrome/test_APIExposer.xhtml
index 4044474df24f..b327abe44d13 100644
--- a/js/xpconnect/tests/chrome/test_APIExposer.xhtml
+++ b/js/xpconnect/tests/chrome/test_APIExposer.xhtml
@@ -29,7 +29,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=634156
 
       function genPropDesc(value) {
           return { enumerable: true, configurable: true, writable: true,
-                   value: value };
+                   value };
       }
       const props = {
           'getProp': genPropDesc(function() { ok(true, "called prop that shadowed a getter"); }),
diff --git a/js/xpconnect/tests/chrome/test_bug1041626.xhtml b/js/xpconnect/tests/chrome/test_bug1041626.xhtml
index 3f970df60d12..61d763083806 100644
--- a/js/xpconnect/tests/chrome/test_bug1041626.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug1041626.xhtml
@@ -42,7 +42,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1041626
     xrayOwnProperties = Object.getOwnPropertyNames(window[0].document);
 
     realOwnProperties = Object.getOwnPropertyNames(window[0].wrappedJSObject.document);
-    ok(realOwnProperties.length > 0);
+    ok(!!realOwnProperties.length);
 
     is(xrayOwnProperties.sort().toSource(), realOwnProperties.sort().toSource(),
        "Xray enumerates document properties properly");
diff --git a/js/xpconnect/tests/chrome/test_bug1074863.html b/js/xpconnect/tests/chrome/test_bug1074863.html
index 7dcad965e563..2fbe69a547b5 100644
--- a/js/xpconnect/tests/chrome/test_bug1074863.html
+++ b/js/xpconnect/tests/chrome/test_bug1074863.html
@@ -12,7 +12,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1074863
   <script type="application/javascript">
 
   /** Test for Bug 1074863 **/
-  var sb = new Cu.Sandbox('http://www.example.com');
+  var sb = new Cu.Sandbox("https://www.example.com");
   sb.namedCtor = Image;
   ok(true, "Didn't assert");
 
diff --git a/js/xpconnect/tests/chrome/test_bug1124898.html b/js/xpconnect/tests/chrome/test_bug1124898.html
index 10b6efe803c0..0b6c2b7451db 100644
--- a/js/xpconnect/tests/chrome/test_bug1124898.html
+++ b/js/xpconnect/tests/chrome/test_bug1124898.html
@@ -21,7 +21,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1124898
     var w = window.browsingContext.topChromeWindow.open("about:blank", "w", "chrome");
     is(w.eval('typeof getAttention'), 'function', 'getAttention exists on regular chrome window');
     is(w.eval('typeof messageManager'), 'object', 'messageManager exists on regular chrome window');
-    var contentURL = "http://example.org/tests/js/xpconnect/tests/mochitest/file_empty.html";
+    var contentURL = "https://example.org/tests/js/xpconnect/tests/mochitest/file_empty.html";
     w.location = contentURL;
     tryWindow();
 
diff --git a/js/xpconnect/tests/chrome/test_bug503926.xhtml b/js/xpconnect/tests/chrome/test_bug503926.xhtml
index 0e1b6184a912..cfe6315fd8b8 100644
--- a/js/xpconnect/tests/chrome/test_bug503926.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug503926.xhtml
@@ -36,7 +36,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=503926
     // Try with a chrome object.
     var passed = false;
     // eslint-disable-next-line mozilla/use-chromeutils-generateqi
-    var obj = { QueryInterface: function() { passed = true; } };
+    var obj = { QueryInterface() { passed = true; } };
     gWindowUtils.xpconnectArgument(obj);
     ok(passed, "trusted QIs should be called");
 
diff --git a/js/xpconnect/tests/chrome/test_bug596580.xhtml b/js/xpconnect/tests/chrome/test_bug596580.xhtml
index 12be1804128e..d0b74f972d3d 100644
--- a/js/xpconnect/tests/chrome/test_bug596580.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug596580.xhtml
@@ -26,11 +26,11 @@
       await SpecialPowers.pushPrefEnv({"set": [["security.allow_eval_with_system_principal",
                                                true]]});
       Test = {
-        include: function(p) {
+        include(p) {
           var sawError = false;
           try {
             Cc["@mozilla.org/moz/jssubscript-loader;1"].
-              getService(Ci["mozIJSSubScriptLoader"]).
+              getService(Ci.mozIJSSubScriptLoader).
               loadSubScript(p);
           } catch (e) {
             sawError = true;
diff --git a/js/xpconnect/tests/chrome/test_bug658909.xhtml b/js/xpconnect/tests/chrome/test_bug658909.xhtml
index 03278d0e3d47..42feb8655893 100644
--- a/js/xpconnect/tests/chrome/test_bug658909.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug658909.xhtml
@@ -31,19 +31,19 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=658909
   }
 
   var testFunctions = {
-    testDocumentElement: function(a, b, name) {
+    testDocumentElement(a, b, name) {
       var getter = Object.prototype.__lookupGetter__.call(a.document, 'documentElement');
       is(getter.call(b.document), b.document.documentElement, msg(a, b, name));
     },
 
-    testInvalidCall: function(a, b, name) {
+    testInvalidCall(a, b, name) {
       var getter = Object.prototype.__lookupGetter__.call(a.document, 'documentElement');
       var threw = false;
       try { getter.call(b.document.body); } catch (e) { threw = true; };
       ok(threw, msg(a, b, name));
     },
 
-    testStatus: function(a, b, name) {
+    testStatus(a, b, name) {
       var setter = Object.prototype.__lookupSetter__.call(a, 'status');
       is(b.status, "", "Empty status");
       setter.call(b, "foopy");
@@ -51,16 +51,16 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=658909
       b.status = "";
     },
 
-    testCreateElement: function(a, b, name) {
+    testCreateElement(a, b, name) {
       is(a.document.createElement.call(b.document, 'div').ownerDocument, b.document, msg(a, b, name));
     },
 
-    testWindowName: function(a, b, name) {
+    testWindowName(a, b, name) {
       var getter = Object.prototype.__lookupGetter__.call(a, 'name');
       is(getter.call(b), b.name, msg(a, b, name));
     },
 
-    testCanvas: function(a, b, name) {
+    testCanvas(a, b, name) {
       var canvasA = a.document.createElement('canvas');
       var canvasB = b.document.createElement('canvas');
       var contextA = canvasA.getContext('2d');
diff --git a/js/xpconnect/tests/chrome/test_bug706301.xhtml b/js/xpconnect/tests/chrome/test_bug706301.xhtml
index bb4f29fa814b..593816b2d4c0 100644
--- a/js/xpconnect/tests/chrome/test_bug706301.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug706301.xhtml
@@ -30,7 +30,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=706301
 
     // Perform the operation.
     Object.getOwnPropertyDescriptor(nodelist, 'length');
-    return nodelist['length'] == 0;
+    return !nodelist.length;
   }
 
   function finishTestInChrome() {
diff --git a/js/xpconnect/tests/chrome/test_bug720619.xhtml b/js/xpconnect/tests/chrome/test_bug720619.xhtml
index 379d34e44c7f..335218886bd2 100644
--- a/js/xpconnect/tests/chrome/test_bug720619.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug720619.xhtml
@@ -18,10 +18,10 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=720619
   <script type="application/javascript"><![CDATA[
   /** Test for Bug 720619 **/
   var obj = {
-    valueOf: function () {
+    valueOf () {
       return 42;
     },
-    toString: function () {
+    toString () {
       return 'str';
     }
   };
diff --git a/js/xpconnect/tests/chrome/test_bug732665.xhtml b/js/xpconnect/tests/chrome/test_bug732665.xhtml
index 521efa644700..ada093c03831 100644
--- a/js/xpconnect/tests/chrome/test_bug732665.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug732665.xhtml
@@ -51,7 +51,7 @@ add_task(async () => {
     return inner();
   }
 
-  var contentSb = new Cu.Sandbox('http://www.example.com');
+  var contentSb = new Cu.Sandbox("https://www.example.com");
   var chromeSb = new Cu.Sandbox(window);
   chromeSb.ok = contentSb.ok = ok;
   Cu.evalInSandbox(nearNativeStackLimit.toSource(), chromeSb);
diff --git a/js/xpconnect/tests/chrome/test_bug732665_meta.js b/js/xpconnect/tests/chrome/test_bug732665_meta.js
index a2afddd98f79..d0187c3830b8 100644
--- a/js/xpconnect/tests/chrome/test_bug732665_meta.js
+++ b/js/xpconnect/tests/chrome/test_bug732665_meta.js
@@ -4,12 +4,13 @@ var top = bottom;
 function nearNativeStackLimit() {
   function inner() {
     try {
-      with ({}) { // keep things predictable -- stay in the interpreter
+      with ({}) {
+        // keep things predictable -- stay in the interpreter
         top = stackPointerInfo();
         var stepsFromLimit = eval("inner()"); // Use eval to force a number of native stackframes to be created.
       }
       return stepsFromLimit + 1;
-    } catch(e) {
+    } catch (e) {
       // It would be nice to check here that the exception is actually an
       // over-recursion here. But doing so would require toString()ing the
       // exception, which we may not have the stack space to do.
@@ -21,6 +22,13 @@ function nearNativeStackLimit() {
 
 var nbFrames = nearNativeStackLimit();
 var frameSize = bottom - top;
-print("Max stack size:", frameSize, "bytes",
-      "\nMaximum number of frames:", nbFrames,
-      "\nAverage frame size:", Math.ceil(frameSize / nbFrames), "bytes");
+print(
+  "Max stack size:",
+  frameSize,
+  "bytes",
+  "\nMaximum number of frames:",
+  nbFrames,
+  "\nAverage frame size:",
+  Math.ceil(frameSize / nbFrames),
+  "bytes"
+);
diff --git a/js/xpconnect/tests/chrome/test_bug760076.xhtml b/js/xpconnect/tests/chrome/test_bug760076.xhtml
index 25e8c401e58f..e3f697355eba 100644
--- a/js/xpconnect/tests/chrome/test_bug760076.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug760076.xhtml
@@ -32,7 +32,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=760076
   }
 
   // Set up the sandbox.
-  var sb = new Cu.Sandbox('http://www.example.com');
+  var sb = new Cu.Sandbox("https://www.example.com");
 
   // Import the functions it needs.
   sb.ok = ok;
diff --git a/js/xpconnect/tests/chrome/test_bug771429.xhtml b/js/xpconnect/tests/chrome/test_bug771429.xhtml
index d2b46101f289..c5846dd6ac10 100644
--- a/js/xpconnect/tests/chrome/test_bug771429.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug771429.xhtml
@@ -27,7 +27,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=771429
       function test(freshCompartment) {
         var s = new Cu.Sandbox(window, {
           sandboxPrototype: window,
-          freshCompartment: freshCompartment,
+          freshCompartment,
         });
         try {
           is(Cu.evalInSandbox('g()', s), window,
diff --git a/js/xpconnect/tests/chrome/test_bug795275.xhtml b/js/xpconnect/tests/chrome/test_bug795275.xhtml
index 1e87c7984c43..18e3c6897a03 100644
--- a/js/xpconnect/tests/chrome/test_bug795275.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug795275.xhtml
@@ -40,13 +40,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=795275
   }
   var gListener = {
     observe: onWarning,
-    QueryInterface: function (iid) {
-      if (!iid.equals(Ci.nsIConsoleListener) &&
-          !iid.equals(Ci.nsISupports)) {
-        throw Cr.NS_ERROR_NO_INTERFACE;
-      }
-      return this;
-    }
+    QueryInterface: ChromeUtils.generateQI(["nsIConsoleListener"])
   };
   var gConsoleService = Cc["@mozilla.org/consoleservice;1"]
                           .getService(Ci.nsIConsoleService);
diff --git a/js/xpconnect/tests/chrome/test_bug799348.xhtml b/js/xpconnect/tests/chrome/test_bug799348.xhtml
index e783da737296..6772ad6461e0 100644
--- a/js/xpconnect/tests/chrome/test_bug799348.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug799348.xhtml
@@ -22,19 +22,18 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=799348
   gCalledOnload = false;
   var myObserver = {
     QueryInterface: ChromeUtils.generateQI(["nsIObserver"]),
-    observe: function(win, topic, data) {
+    observe(win, topic, data) {
       if (topic == "domwindowopened") {
         ok(!gCalledOnload, "domwindowopened notification fired before onload");
         win.addEventListener("load", function(evt) {
           gCalledOnload = true;
           win.close();
-        }, false);
+        });
       } else if (topic == "domwindowclosed") {
         ok(gCalledOnload, "should have called onload");
         Services.ww.unregisterNotification(myObserver);
         SimpleTest.finish();
-      }
-      else {
+      } else {
         ok(false, "unknown topic");
       }
     }
diff --git a/js/xpconnect/tests/chrome/test_bug895340.xhtml b/js/xpconnect/tests/chrome/test_bug895340.xhtml
index eba381bd7bc5..14406949acda 100644
--- a/js/xpconnect/tests/chrome/test_bug895340.xhtml
+++ b/js/xpconnect/tests/chrome/test_bug895340.xhtml
@@ -27,14 +27,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=895340
                   .getService(Ci.nsIConsoleService);
 
   let listener = {
-    QueryInterface: function(iid) {
-      if (!iid.equals(Ci.nsIConsoleListener) &&
-          !iid.equals(Ci.nsISupports))
-      {
-        throw Cr.NS_ERROR_NO_INTERFACE;
-      }
-      return this;
-    }
+    QueryInterface: ChromeUtils.generateQI(["nsIConsoleListener"])
   };
 
   listener.observe = function(aMessage) {
diff --git a/js/xpconnect/tests/chrome/test_cloneInto.xhtml b/js/xpconnect/tests/chrome/test_cloneInto.xhtml
index bf810261947a..03d4b3f9ca57 100644
--- a/js/xpconnect/tests/chrome/test_cloneInto.xhtml
+++ b/js/xpconnect/tests/chrome/test_cloneInto.xhtml
@@ -45,10 +45,10 @@
     if (Array.isArray(a))
       return 'array';
 
-    if (a instanceof File)
+    if (File.isInstance(a))
       return 'file';
 
-    if (a instanceof Blob)
+    if (Blob.isInstance(a))
       return 'blob';
 
     if (TypedArrayThings.includes(a.constructor.name))
@@ -152,7 +152,7 @@
     cloneAndTest(tests[i]);
   }
 
-  checkThrows(function() { Cu.cloneInto({ a: function() {} }, sandbox); },
+  checkThrows(function() { Cu.cloneInto({ a() {} }, sandbox); },
               'Function should not be cloned by default');
 
   checkThrows(function() { Cu.cloneInto({ a: document }, sandbox); },
@@ -181,11 +181,11 @@
               'wrapReflectors may not wrap cross-origin reflectors', /unsupported value type/);
 
 
-  var test = { a: function() { return 42; } };
+  var test = { a() { return 42; } };
   cloneAndTestWithFunctions(test);
 
   // Check that inputs are properly passed through cloned functions:
-  test = { a: function(obj) { return obj; } };
+  test = { a(obj) { return obj; } };
   var clonedTest = Cu.cloneInto(test, sandbox, {cloneFunctions: true});
   var testInput = {};
   is(clonedTest.a(testInput), testInput, "Objects should be identical");
diff --git a/js/xpconnect/tests/chrome/test_exposeInDerived.xhtml b/js/xpconnect/tests/chrome/test_exposeInDerived.xhtml
index b4594be7fbd0..d1cac9194104 100644
--- a/js/xpconnect/tests/chrome/test_exposeInDerived.xhtml
+++ b/js/xpconnect/tests/chrome/test_exposeInDerived.xhtml
@@ -19,7 +19,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=804630
   <![CDATA[
   /** Test to make sure that COWed objects can't expose properties from their prototypes. **/
   // Set up the sandbox.
-  var sb = new Cu.Sandbox('http://www.example.com');
+  var sb = new Cu.Sandbox("https://www.example.com");
   sb.ok = ok;
   sb.is = is;
 
diff --git a/js/xpconnect/tests/chrome/test_weakmap_keys_preserved2.xhtml b/js/xpconnect/tests/chrome/test_weakmap_keys_preserved2.xhtml
index d93bc2d30c65..b97db7acda4f 100644
--- a/js/xpconnect/tests/chrome/test_weakmap_keys_preserved2.xhtml
+++ b/js/xpconnect/tests/chrome/test_weakmap_keys_preserved2.xhtml
@@ -48,7 +48,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=673468
     sandbox.wrapper = null;
     sandbox.value = null;
 
-    live_dom.xyzabc = {wrappee: wrappee, m: map, sb: sandbox};
+    live_dom.xyzabc = {wrappee, m: map, sb: sandbox};
 
     let key = ChromeUtils.nondeterministicGetWeakMapKeys(map)[0];
     let value = map.get(key);
diff --git a/js/xpconnect/tests/chrome/test_windowProxyDeadWrapper.html b/js/xpconnect/tests/chrome/test_windowProxyDeadWrapper.html
index 9d8a2a77618d..7970c1ae70dc 100644
--- a/js/xpconnect/tests/chrome/test_windowProxyDeadWrapper.html
+++ b/js/xpconnect/tests/chrome/test_windowProxyDeadWrapper.html
@@ -33,7 +33,7 @@ async function go() {
     ok(!checkDead(), "WindowProxy shouldn't be dead yet");
 
     // Load a content page in the chrome frame.
-    w.location = "http://example.org/tests/js/xpconnect/tests/mochitest/file_empty.html";
+    w.location = "https://example.org/tests/js/xpconnect/tests/mochitest/file_empty.html";
     tryWindow();
 
     function tryWindow() {
diff --git a/js/xpconnect/tests/chrome/test_xrayToJS.xhtml b/js/xpconnect/tests/chrome/test_xrayToJS.xhtml
index a3fb24aa470b..797f89e97170 100644
--- a/js/xpconnect/tests/chrome/test_xrayToJS.xhtml
+++ b/js/xpconnect/tests/chrome/test_xrayToJS.xhtml
@@ -77,7 +77,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
       is(Cu.unwaiveXrays(Cu.waiveXrays(new iwin[c](...args)).constructor), iwin[c],
          "constructor property is set up right: " + c);
       let expectedProto = Cu.isOpaqueWrapper(new iwin[c](...args)) ?
-        iwin['Object'].prototype : iwin[c].prototype;
+        iwin.Object.prototype : iwin[c].prototype;
       is(Object.getPrototypeOf(new iwin[c](...args)), expectedProto,
          "prototype is correct: " + c);
       is(global(new iwin[c](...args)), iwin, "Got the right global: " + c);
@@ -147,7 +147,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
       is(Cu.unwaiveXrays(Cu.waiveXrays(new iwin.AggregateError(...args)).constructor), iwin.AggregateError,
          "AggregateError constructor property is set up right");
       let expectedProto =  Cu.isOpaqueWrapper(new iwin.AggregateError(...args)) ?
-        iwin['Object'].prototype : iwin.AggregateError.prototype;
+        iwin.Object.prototype : iwin.AggregateError.prototype;
       is(Object.getPrototypeOf(new iwin.AggregateError(...args)), expectedProto,
          "AggregateError prototype is correct");
       is(global(new iwin.AggregateError(...args)), iwin, "Got the right global for AggregateError");
@@ -203,7 +203,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
     // Some props live on all constructors
     return arr.concat(["prototype", "length", "name"]);
   }
-  gPrototypeProperties['Date'] =
+  gPrototypeProperties.Date =
     ["getTime", "getTimezoneOffset", "getYear", "getFullYear", "getUTCFullYear",
     "getMonth", "getUTCMonth", "getDate", "getUTCDate", "getDay", "getUTCDay",
     "getHours", "getUTCHours", "getMinutes", "getUTCMinutes", "getSeconds",
@@ -215,36 +215,36 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
     "toLocaleDateString", "toLocaleTimeString", "toDateString", "toTimeString",
     "toISOString", "toJSON", "toSource", "toString", "valueOf", "constructor",
     "toGMTString", Symbol.toPrimitive];
-  gConstructorProperties['Date'] = constructorProps(["UTC", "parse", "now"]);
-  gPrototypeProperties['Object'] =
+  gConstructorProperties.Date = constructorProps(["UTC", "parse", "now"]);
+  gPrototypeProperties.Object =
     ["constructor", "toSource", "toString", "toLocaleString", "valueOf",
      "hasOwnProperty", "isPrototypeOf", "propertyIsEnumerable",
      "__defineGetter__", "__defineSetter__", "__lookupGetter__", "__lookupSetter__",
      "__proto__"];
-  gConstructorProperties['Object'] =
+  gConstructorProperties.Object =
     constructorProps(["setPrototypeOf", "getOwnPropertyDescriptor", "getOwnPropertyDescriptors",
                       "keys", "is", "defineProperty", "defineProperties", "create",
                       "getOwnPropertyNames", "getOwnPropertySymbols",
                       "preventExtensions", "freeze", "fromEntries", "isFrozen", "seal",
                       "isSealed", "assign", "getPrototypeOf", "values",
                       "entries", "isExtensible", "hasOwn"]);
-  gPrototypeProperties['Array'] =
+  gPrototypeProperties.Array =
     ["length", "toSource", "toString", "toLocaleString", "join", "reverse", "sort", "push",
       "pop", "shift", "unshift", "splice", "concat", "slice", "lastIndexOf", "indexOf",
       "includes", "forEach", "map", "reduce", "reduceRight", "filter", "some", "every", "find",
       "findIndex", "copyWithin", "fill", Symbol.iterator, Symbol.unscopables, "entries", "keys",
       "values", "constructor", "flat", "flatMap", "at", "findLast", "findLastIndex"];
   if (isNightlyBuild) {
-    gPrototypeProperties['Array'].push("group");
-    gPrototypeProperties['Array'].push("groupToMap");
+    gPrototypeProperties.Array.push("group");
+    gPrototypeProperties.Array.push("groupToMap");
   }
-  gConstructorProperties['Array'] =
+  gConstructorProperties.Array =
     constructorProps(["isArray", "from", "of", Symbol.species]);
   for (var c of typedArrayClasses) {
     gPrototypeProperties[c] = ["constructor", "BYTES_PER_ELEMENT"];
     gConstructorProperties[c] = constructorProps(["BYTES_PER_ELEMENT"]);
   }
-  gPrototypeProperties['TypedArray'] =
+  gPrototypeProperties.TypedArray =
     ["length", "buffer", "byteLength", "byteOffset", Symbol.iterator, "subarray",
      "set", "copyWithin", "find", "findIndex", "forEach","indexOf", "lastIndexOf", "includes",
      "reverse", "join", "every", "some", "reduce", "reduceRight", "entries", "keys", "values",
@@ -256,68 +256,68 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
       gConstructorProperties[c] = constructorProps([]);
   }
   // toString and toSource only live on the parent proto (Error.prototype).
-  gPrototypeProperties['Error'].push('toString');
-  gPrototypeProperties['Error'].push('toSource');
+  gPrototypeProperties.Error.push('toString');
+  gPrototypeProperties.Error.push('toSource');
 
-  gPrototypeProperties['Function'] =
+  gPrototypeProperties.Function =
     ["constructor", "toSource", "toString", "apply", "call", "bind",
      "length", "name", "arguments", "caller", Symbol.hasInstance];
-  gConstructorProperties['Function'] = constructorProps([])
+  gConstructorProperties.Function = constructorProps([])
 
-  gPrototypeProperties['RegExp'] =
+  gPrototypeProperties.RegExp =
     ["constructor", "toSource", "toString", "compile", "exec", "test",
      Symbol.match, Symbol.matchAll, Symbol.replace, Symbol.search, Symbol.split,
      "flags", "dotAll", "global", "hasIndices", "ignoreCase", "multiline", "source", "sticky",
      "unicode"];
-  gConstructorProperties['RegExp'] =
+  gConstructorProperties.RegExp =
     constructorProps(["input", "lastMatch", "lastParen",
                       "leftContext", "rightContext", "$1", "$2", "$3", "$4",
                       "$5", "$6", "$7", "$8", "$9", "$_", "$&", "$+",
                       "$`", "$'", Symbol.species])
 
-  gPrototypeProperties['Promise'] =
+  gPrototypeProperties.Promise =
     ["constructor", "catch", "then", "finally", Symbol.toStringTag];
-  gConstructorProperties['Promise'] =
+  gConstructorProperties.Promise =
     constructorProps(["resolve", "reject", "all", "allSettled", "any", "race", Symbol.species]);
 
-  gPrototypeProperties['ArrayBuffer'] =
+  gPrototypeProperties.ArrayBuffer =
     ["constructor", "byteLength", "slice", Symbol.toStringTag];
-  gConstructorProperties['ArrayBuffer'] =
+  gConstructorProperties.ArrayBuffer =
     constructorProps(["isView", Symbol.species]);
 
-  gPrototypeProperties['SharedArrayBuffer'] = ["constructor", "slice", "byteLength", Symbol.toStringTag];
-  gConstructorProperties['SharedArrayBuffer'] = constructorProps([Symbol.species]);
+  gPrototypeProperties.SharedArrayBuffer = ["constructor", "slice", "byteLength", Symbol.toStringTag];
+  gConstructorProperties.SharedArrayBuffer = constructorProps([Symbol.species]);
 
-  gPrototypeProperties['Map'] =
+  gPrototypeProperties.Map =
     ["constructor", "size", Symbol.toStringTag, "get", "has", "set", "delete",
      "keys", "values", "clear", "forEach", "entries", Symbol.iterator];
-  gConstructorProperties['Map'] =
+  gConstructorProperties.Map =
     constructorProps([Symbol.species]);
 
-  gPrototypeProperties['Set'] =
+  gPrototypeProperties.Set =
     ["constructor", "size", Symbol.toStringTag, "has", "add", "delete",
      "keys", "values", "clear", "forEach", "entries", Symbol.iterator];
-  gConstructorProperties['Set'] =
+  gConstructorProperties.Set =
     constructorProps([Symbol.species]);
 
-  gPrototypeProperties['WeakMap'] =
+  gPrototypeProperties.WeakMap =
     ["constructor", Symbol.toStringTag, "get", "has", "set", "delete"];
-  gConstructorProperties['WeakMap'] =
+  gConstructorProperties.WeakMap =
     constructorProps([]);
 
-  gPrototypeProperties['WeakSet'] =
+  gPrototypeProperties.WeakSet =
     ["constructor", Symbol.toStringTag, "has", "add", "delete"];
-  gConstructorProperties['WeakSet'] =
+  gConstructorProperties.WeakSet =
     constructorProps([]);
 
-  gPrototypeProperties['DataView'] =
+  gPrototypeProperties.DataView =
     ["constructor", "buffer", "byteLength", "byteOffset", Symbol.toStringTag,
      "getInt8", "getUint8", "getInt16", "getUint16",
      "getInt32", "getUint32", "getFloat32", "getFloat64",
      "setInt8", "setUint8", "setInt16", "setUint16",
      "setInt32", "setUint32", "setFloat32", "setFloat64",
      "getBigInt64", "getBigUint64", "setBigInt64", "setBigUint64"]
-  gConstructorProperties['DataView'] = constructorProps([]);
+  gConstructorProperties.DataView = constructorProps([]);
 
   // Sort an array that may contain symbols as well as strings.
   function sortProperties(arr) {
@@ -371,7 +371,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
       is(lookupCallable(xray), method, "Proto props resolve on the instance");
       let local = lookupCallable(localProto);
       is(method.length, local.length, "Function.length identical");
-      if (method.length == 0) {
+      if (!method.length) {
         is(method.call(xray) + "", local.call(xray) + "",
            "Xray and local method results stringify identically");
 
@@ -426,7 +426,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
       // they live in, as far as I can tell, so testing them over Xrays will be
       // wrong: on the Xray they will actaully get the lastMatch of _our_
       // global, not the Xrayed one.
-      if (method.length == 0 &&
+      if (!method.length &&
           !(localCtor.name == "Date" && name == "now") &&
           !(localCtor.name == "RegExp" && (name == "lastMatch" || name == "$&"))) {
         is(method.call(xrayCtor) + "", local.call(xrayCtor) + "",
@@ -457,7 +457,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
     let protoCallables = protoProps.filter(name => propertyIsGetter(localProto, name, classname) ||
                                                    typeof localProto[name] == 'function' &&
                                                    name != 'constructor');
-    ok(protoCallables.length > 0, "Need something to test");
+    ok(!!protoCallables.length, "Need something to test");
     is(xrayProto, iwin[classname].prototype, "Xray proto is correct");
     is(xrayProto, xray.__proto__, "Proto accessors agree");
     var protoProto = classname == "Object" ? null : iwin.Object.prototype;
@@ -777,7 +777,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=933681
     checkThrows(function() { trickyObject.hasOwnProperty = 33; }, /shadow/,
                 "Should reject shadowing of pre-existing inherited properties over Xrays");
 
-    checkThrows(function() { Object.defineProperty(trickyObject, 'rejectedProp', { get: function() {}}); },
+    checkThrows(function() { Object.defineProperty(trickyObject, 'rejectedProp', { get() {}}); },
                 /accessor property/, "Should reject accessor property definition");
   }
 
diff --git a/js/xpconnect/tests/chrome/utf8_subscript.js b/js/xpconnect/tests/chrome/utf8_subscript.js
index d4051b1a85cd..6444273c9d90 100644
--- a/js/xpconnect/tests/chrome/utf8_subscript.js
+++ b/js/xpconnect/tests/chrome/utf8_subscript.js
@@ -1,3 +1,5 @@
 // -*- coding: utf-8; indent-tabs-mode: nil -*-
 var str = "𝔘𝔫𝔦𝔠𝔬𝔡𝔢";
-function f() { return 42; }
+function f() {
+  return 42;
+}