diff --git a/testing/mochitest/browser-harness.xul b/testing/mochitest/browser-harness.xul
index 77390327d833..f968bdd1f9e3 100644
--- a/testing/mochitest/browser-harness.xul
+++ b/testing/mochitest/browser-harness.xul
@@ -110,6 +110,7 @@
     function browserTest(aTestFile) {
       this.path = aTestFile['url'];
       this.expected = aTestFile['expected'];
+      this.usesUnsafeCPOWs = aTestFile['uses-unsafe-cpows'] || false;
       this.dumper = gDumper;
       this.results = [];
       this.scope = null;
diff --git a/testing/mochitest/browser-test.js b/testing/mochitest/browser-test.js
index 5ecf1b8729dc..cd1e78d1acad 100644
--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -379,6 +379,16 @@ function Tester(aTests, structuredLogger, aCallback) {
   this._scriptLoader = Services.scriptloader;
   this.EventUtils = {};
   this._scriptLoader.loadSubScript("chrome://mochikit/content/tests/SimpleTest/EventUtils.js", this.EventUtils);
+
+  // In order to allow existing tests to continue using unsafe CPOWs
+  // with EventUtils, we need to load a separate copy into a sandbox
+  // which has unsafe CPOW usage whitelisted.
+  this.cpowSandbox = Cu.Sandbox(window, {sandboxPrototype: window});
+  Cu.permitCPOWsInScope(this.cpowSandbox);
+
+  this.cpowEventUtils = new this.cpowSandbox.Object();
+  this._scriptLoader.loadSubScript("chrome://mochikit/content/tests/SimpleTest/EventUtils.js", this.cpowEventUtils);
+
   var simpleTestScope = {};
   this._scriptLoader.loadSubScript("chrome://mochikit/content/tests/SimpleTest/specialpowersAPI.js", simpleTestScope);
   this._scriptLoader.loadSubScript("chrome://mochikit/content/tests/SimpleTest/SpecialPowersObserverAPI.js", simpleTestScope);
@@ -970,16 +980,17 @@ Tester.prototype = {
     let currentTest = this.currentTest;
 
     // Import utils in the test scope.
-    this.currentTest.scope.EventUtils = this.EventUtils;
-    this.currentTest.scope.SimpleTest = this.SimpleTest;
-    this.currentTest.scope.gTestPath = this.currentTest.path;
-    this.currentTest.scope.Task = this.Task;
-    this.currentTest.scope.ContentTask = this.ContentTask;
-    this.currentTest.scope.BrowserTestUtils = this.BrowserTestUtils;
-    this.currentTest.scope.TestUtils = this.TestUtils;
-    this.currentTest.scope.ExtensionTestUtils = this.ExtensionTestUtils;
+    let {scope} = this.currentTest;
+    scope.EventUtils = this.currentTest.usesUnsafeCPOWs ? this.cpowEventUtils : this.EventUtils;
+    scope.SimpleTest = this.SimpleTest;
+    scope.gTestPath = this.currentTest.path;
+    scope.Task = this.Task;
+    scope.ContentTask = this.ContentTask;
+    scope.BrowserTestUtils = this.BrowserTestUtils;
+    scope.TestUtils = this.TestUtils;
+    scope.ExtensionTestUtils = this.ExtensionTestUtils;
     // Pass a custom report function for mochitest style reporting.
-    this.currentTest.scope.Assert = new this.Assert(function(err, message, stack) {
+    scope.Assert = new this.Assert(function(err, message, stack) {
       currentTest.addResult(new testResult(err ? {
         name: err.message,
         ex: err.stack,
@@ -996,7 +1007,7 @@ Tester.prototype = {
     this.ContentTask.setTestScope(currentScope);
 
     // Allow Assert.jsm methods to be tacked to the current scope.
-    this.currentTest.scope.export_assertions = function() {
+    scope.export_assertions = function() {
       for (let func in this.Assert) {
         this[func] = this.Assert[func].bind(this.Assert);
       }
@@ -1005,11 +1016,11 @@ Tester.prototype = {
     // Override SimpleTest methods with ours.
     SIMPLETEST_OVERRIDES.forEach(function(m) {
       this.SimpleTest[m] = this[m];
-    }, this.currentTest.scope);
+    }, scope);
 
     //load the tools to work with chrome .jar and remote
     try {
-      this._scriptLoader.loadSubScript("chrome://mochikit/content/chrome-harness.js", this.currentTest.scope);
+      this._scriptLoader.loadSubScript("chrome://mochikit/content/chrome-harness.js", scope);
     } catch (ex) { /* no chrome-harness tools */ }
 
     // Import head.js script if it exists.
@@ -1017,7 +1028,7 @@ Tester.prototype = {
       this.currentTest.path.substr(0, this.currentTest.path.lastIndexOf("/"));
     var headPath = currentTestDirPath + "/head.js";
     try {
-      this._scriptLoader.loadSubScript(headPath, this.currentTest.scope);
+      this._scriptLoader.loadSubScript(headPath, scope);
     } catch (ex) {
       // Ignore if no head.js exists, but report all other errors.  Note this
       // will also ignore an existing head.js attempting to import a missing
@@ -1032,8 +1043,7 @@ Tester.prototype = {
 
     // Import the test script.
     try {
-      this._scriptLoader.loadSubScript(this.currentTest.path,
-                                       this.currentTest.scope);
+      this._scriptLoader.loadSubScript(this.currentTest.path, scope);
       // Run the test
       this.lastStartTime = Date.now();
       if (this.currentTest.scope.__tasks) {
@@ -1091,8 +1101,8 @@ Tester.prototype = {
           }
           this.finish();
         }.bind(currentScope));
-      } else if (typeof this.currentTest.scope.test == "function") {
-        this.currentTest.scope.test();
+      } else if (typeof scope.test == "function") {
+        scope.test();
       } else {
         throw "This test didn't call add_task, nor did it define a generatorTest() function, nor did it define a test() function, so we don't know how to run it.";
       }
@@ -1372,6 +1382,18 @@ function testScope(aTester, aTest, expected) {
       self.__tester.structuredLogger.activateBuffering();
     })
   };
+
+  // If we're running a test that requires unsafe CPOWs, create a
+  // separate sandbox scope, with CPOWS whitelisted, for that test, and
+  // mirror all of our properties onto it. Test files will be loaded
+  // into this sandbox.
+  //
+  // Otherwise, load test files directly into the testScope instance.
+  if (aTest.usesUnsafeCPOWs) {
+    let sandbox = this._createSandbox();
+    Cu.permitCPOWsInScope(sandbox);
+    return sandbox;
+  }
 }
 
 function decorateTaskFn(fn) {
@@ -1400,6 +1422,29 @@ testScope.prototype = {
   ExtensionTestUtils: null,
   Assert: null,
 
+  _createSandbox() {
+    let sandbox = Cu.Sandbox(window, {sandboxPrototype: window});
+
+    for (let prop in this) {
+      if (typeof this[prop] == "function") {
+        sandbox[prop] = this[prop].bind(this);
+      } else {
+        Object.defineProperty(sandbox, prop, {
+          configurable: true,
+          enumerable: true,
+          get: () => {
+            return this[prop];
+          },
+          set: (value) => {
+            this[prop] = value;
+          }
+        });
+      }
+    }
+
+    return sandbox;
+  },
+
   /**
    * Add a test function which is a Task function.
    *
diff --git a/testing/mochitest/manifestLibrary.js b/testing/mochitest/manifestLibrary.js
index 8611a9e22d8f..c2df2d669e81 100644
--- a/testing/mochitest/manifestLibrary.js
+++ b/testing/mochitest/manifestLibrary.js
@@ -26,10 +26,10 @@ function parseTestManifest(testManifest, params, callback) {
     }
     if (params.testRoot != 'tests' && params.testRoot !== undefined) {
       name = params.baseurl + '/' + params.testRoot + '/' + path;
-      links[name] = {'test': {'url': name, 'expected': obj['expected']}};
+      links[name] = {'test': {'url': name, 'expected': obj['expected'], 'uses-unsafe-cpows': obj['uses-unsafe-cpows']}};
     } else {
       name = params.testPrefix + path;
-      paths.push({'test': {'url': name, 'expected': obj['expected']}});
+      paths.push({'test': {'url': name, 'expected': obj['expected'], 'uses-unsafe-cpows': obj['uses-unsafe-cpows']}});
     }
   }
   if (paths.length > 0) {
diff --git a/testing/mochitest/runtests.py b/testing/mochitest/runtests.py
index 66066b00c1de..4f81a3a77c1c 100644
--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -1524,6 +1524,8 @@ toolbar#nav-bar {
                 testob['disabled'] = test['disabled']
             if 'expected' in test:
                 testob['expected'] = test['expected']
+            if 'uses-unsafe-cpows' in test:
+                testob['uses-unsafe-cpows'] = test['uses-unsafe-cpows'] == 'true'
             if 'scheme' in test:
                 testob['scheme'] = test['scheme']
             if options.failure_pattern_file:
diff --git a/testing/mochitest/tests/SimpleTest/EventUtils.js b/testing/mochitest/tests/SimpleTest/EventUtils.js
index 8d6fb0005e9d..1772edb7bdf7 100644
--- a/testing/mochitest/tests/SimpleTest/EventUtils.js
+++ b/testing/mochitest/tests/SimpleTest/EventUtils.js
@@ -176,6 +176,10 @@ function sendMouseEvent(aEvent, aTarget, aWindow) {
                        ctrlKeyArg, altKeyArg, shiftKeyArg, metaKeyArg,
                        buttonArg, relatedTargetArg);
 
+  // If documentURIObject exists or `window` is a stub object, we're in
+  // a chrome scope, so don't bother trying to go through SpecialPowers.
+  if (!window.document || window.document.documentURIObject)
+    return aTarget.dispatchEvent(event);
   return SpecialPowers.dispatchEvent(aWindow, aTarget, event);
 }
 
@@ -1240,6 +1244,14 @@ function _getDOMWindowUtils(aWindow = window)
     aWindow = window;
   }
 
+  // If documentURIObject exists or `window` is a stub object, we're in
+  // a chrome scope, so don't bother trying to go through SpecialPowers.
+  if (!window.document || window.document.documentURIObject) {
+    return aWindow
+        .QueryInterface(_EU_Ci.nsIInterfaceRequestor)
+        .getInterface(_EU_Ci.nsIDOMWindowUtils);
+  }
+
   // we need parent.SpecialPowers for:
   //  layout/base/tests/test_reftests_with_caret.html
   //  chrome: toolkit/content/tests/chrome/test_findbar.xul