--- a/dom/tests/mochitest/bugs/test_bug1171215.html
+++ b/dom/tests/mochitest/bugs/test_bug1171215.html
@@ -23,17 +23,17 @@ https://bugzilla.mozilla.org/show_bug.cg
 
     // Set a cookie in example.org so we can test that we can't read it in
     // third-party cases.
     f.contentWindow.location =
         "http://example.org/tests/dom/tests/mochitest/bugs/file_prime_cookie.html";
     waitForLoad().then(function() {
         // Cookies are set up, disallow third-party cookies and start the test.
         SpecialPowers.pushPrefEnv({ set: [[ 'network.cookie.cookieBehavior', 1 ]] },
-                                  () => { spawn_task(continueTest); });
+                                  () => { continueTest(); });
     }).catch((e) => { ok(false, `Got exception: ${e}`) });
   }
 
   function waitForLoad() {
     return new Promise((resolve) => {
       window.addEventListener("message", function(msg) {
         info(`got message ${msg.data}`);
         resolve(msg.data);

--- a/editor/libeditor/tests/test_selection_move_commands.html
+++ b/editor/libeditor/tests/test_selection_move_commands.html
@@ -8,18 +8,18 @@
 <iframe id="edit" width="200" height="100" src="about:blank"></iframe>
 
 <script>
 SimpleTest.waitForExplicitFinish();
 SimpleTest.requestFlakyTimeout("Legacy test, possibly no good reason");
 
 var winUtils = SpecialPowers.getDOMWindowUtils(window);
 
-function* setup() {
-  yield SpecialPowers.pushPrefEnv({set: [["general.smoothScroll", false]]});
+async function setup() {
+  await SpecialPowers.pushPrefEnv({set: [["general.smoothScroll", false]]});
   winUtils.advanceTimeAndRefresh(100);
 }
 
 function* runTests() {
   var e = document.getElementById("edit");
   var doc = e.contentDocument;
   var win = e.contentWindow;
   var root = doc.documentElement;
@@ -200,13 +200,13 @@ async function testRunner() {
     if (curTest.next().done) {
       break;
     }
     winUtils.advanceTimeAndRefresh(100);
     await new Promise(resolve => setTimeout(resolve, 20));
   }
 }
 
-spawn_task(setup)
-  .then(() => spawn_task(testRunner))
-  .then(() => spawn_task(cleanup))
+setup()
+  .then(() => testRunner())
+  .then(() => cleanup())
   .catch(err => ok(false, err));
 </script>

--- a/layout/style/test/chrome/bug418986-2.js
+++ b/layout/style/test/chrome/bug418986-2.js
@@ -281,18 +281,17 @@ var testOSXFontSmoothing = function (res
 var sleep = function (timeoutMs) {
   return new Promise(function(resolve, reject) {
     window.setTimeout(resolve);
   });
 };
 
 // __testMediaQueriesInPictureElements(resisting)__.
 // Test to see if media queries are properly spoofed in picture elements
-// when we are resisting fingerprinting. A generator function
-// to be used with SpawnTask.js.
+// when we are resisting fingerprinting.
 var testMediaQueriesInPictureElements = async function(resisting) {
   let picture = document.createElementNS(HTML_NS, "picture");
   for (let [key, offVal, onVal] of expected_values) {
     let expected = resisting ? onVal : offVal;
     if (expected) {
       let query = constructQuery(key, expected);
 
       let source = document.createElementNS(HTML_NS, "source");
@@ -322,18 +321,17 @@ var testMediaQueriesInPictureElements = 
 // when it succeeds.
 var pushPref = function (key, value) {
   return new Promise(function(resolve, reject) {
     SpecialPowers.pushPrefEnv({"set": [[key, value]]}, resolve);
   });
 };
 
 // __test(isContent)__.
-// Run all tests. A generator function to be used
-// with SpawnTask.js.
+// Run all tests.
 var test = async function(isContent) {
   for (prefValue of [false, true]) {
     await pushPref("privacy.resistFingerprinting", prefValue);
     let resisting = prefValue && isContent;
     expected_values.forEach(
       function ([key, offVal, onVal]) {
         testMatch(key, resisting ? onVal : offVal);
       });

--- a/layout/style/test/chrome/test_bug418986-2.xul
+++ b/layout/style/test/chrome/test_bug418986-2.xul
@@ -17,14 +17,14 @@ https://bugzilla.mozilla.org/show_bug.cg
   <p id="pictures"></p>
   </body>
 
   <script type="text/javascript" src="bug418986-2.js"></script>
   <!-- test code goes here -->
   <script type="text/javascript">
     // Run all tests now.
     window.onload = function () {
-      add_task(function* () {
-        yield test(false);
+      add_task(async function() {
+        await test(false);
       });
     };
   </script>
 </window>

--- a/testing/mochitest/chrome/chrome.ini
+++ b/testing/mochitest/chrome/chrome.ini
@@ -1,18 +1,16 @@
 [DEFAULT]
 skip-if = os == 'android'
 support-files = test-dir/test-file
 
 [test_sample.xul]
-[test_sanityAddTask.xul]
 [test_sanityEventUtils.xul]
 [test_sanityPluginUtils.html]
 [test_sanityException.xul]
 [test_sanityException2.xul]
 [test_sanityManifest.xul]
 fail-if = true
 [test_sanityManifest_pf.xul]
 fail-if = true
-[test_sanitySpawnTask.xul]
 [test_chromeGetTestFile.xul]
 [test_tasks_skip.xul]
 [test_tasks_skipall.xul]

deleted file mode 100644
--- a/testing/mochitest/chrome/test_sanityAddTask.xul
+++ /dev/null
@@ -1,43 +0,0 @@
-<?xml version="1.0"?>
-<!-- This Source Code Form is subject to the terms of the Mozilla Public
-   - License, v. 2.0. If a copy of the MPL was not distributed with this
-   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
-<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css"
-                 type="text/css"?>
-<window title="Test spawnTawk function"
-        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
-  <script type="application/javascript"
-          src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
-  <script type="application/javascript"
-          src="chrome://mochikit/content/tests/SimpleTest/SpawnTask.js"/>
-  <script type="application/javascript">
-    <![CDATA[
-
-      // Check that we can 'add_task' a few times and all tasks run asynchronously before test finishes.
-
-      add_task(async function() {
-        var x = await Promise.resolve(1);
-        is(x, 1, "task yields Promise value as expected");
-      });
-
-      add_task(function* () {
-        var x = yield [Promise.resolve(1), Promise.resolve(2), Promise.resolve(3)];
-        is(x.join(""), "123", "task yields Promise value as expected");
-      });
-
-      add_task(function* () {
-        var x = yield (function* () {
-          return 3;
-        }());
-        is(x, 3, "task yields generator function return value as expected");
-      });
-
-    ]]>
-  </script>
-  <body xmlns="http://www.w3.org/1999/xhtml" >
-  </body>
-</window>
-
-
-
-

deleted file mode 100644
--- a/testing/mochitest/chrome/test_sanitySpawnTask.xul
+++ /dev/null
@@ -1,70 +0,0 @@
-<?xml version="1.0"?>
-<!-- This Source Code Form is subject to the terms of the Mozilla Public
-   - License, v. 2.0. If a copy of the MPL was not distributed with this
-   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
-<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css"
-                 type="text/css"?>
-<window title="Test spawnTawk function"
-        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
-  <script type="application/javascript"
-          src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"/>
-  <script type="application/javascript"
-          src="chrome://mochikit/content/tests/SimpleTest/SpawnTask.js"/>
-  <script type="application/javascript">
-  <![CDATA[
-    SimpleTest.waitForExplicitFinish();
-
-    var externalGeneratorFunction = function* () {
-      return 8;
-    };
-
-    var nestedFunction = function* () {
-      return yield function* () {
-        return yield function* () {
-          return yield function* () {
-            return yield Promise.resolve(9);
-          }();
-        }();
-      }();
-    }
-
-    var variousTests = function* () {
-      var val1 = yield [Promise.resolve(1), Promise.resolve(2), Promise.resolve(3)];
-      is(val1.join(""), "123", "Array of promises -> Promise.all");
-      var val2 = yield Promise.resolve(2);
-      is(val2, 2, "Resolved promise yields value.");
-      var val3 = yield function* () { return 3; };
-      is(val3, 3, "Generator functions are spawned.");
-      //var val4 = yield function () { return 4; };
-      //is(val4, 4, "Plain functions run and return.");
-      var val5 = yield (function* () { return 5; }());
-      is(val5, 5, "Generators are spawned.");
-      try {
-        var val6 = yield Promise.reject(Error("error6"));
-        ok(false, "Shouldn't reach this line.");
-      } catch (error) {
-        is(error.message, "error6", "Rejected promise throws error.");
-      }
-      try {
-        var val7 = yield function* () { throw Error("error7"); };
-        ok(false, "Shouldn't reach this line.");
-      } catch (error) {
-        is(error.message, "error7", "Thrown error propagates.");
-      }
-      var val8 = yield externalGeneratorFunction();
-      is(val8, 8, "External generator also spawned.");
-      var val9 = yield nestedFunction();
-      is(val9, 9, "Nested generator functions work.");
-      return 10;
-    };
-
-    spawn_task(variousTests).then(function(result) {
-      is(result, 10, "spawn_task(...) returns promise");
-      SimpleTest.finish();
-    });
-    ]]>
-  </script>
-
-  <body xmlns="http://www.w3.org/1999/xhtml" >
-  </body>
-</window>

--- a/testing/mochitest/tests/Harness_sanity/mochitest.ini
+++ b/testing/mochitest/tests/Harness_sanity/mochitest.ini
@@ -1,12 +1,11 @@
 [DEFAULT]
 [test_TestsRunningAfterSimpleTestFinish.html]
 skip-if = true #depends on fix for bug 1048446
-[test_add_task.html]
 [test_createFiles.html]
 [test_importInMainProcess.html]
 support-files = importtesting_chromescript.js
 [test_sanity.html]
 [test_sanityException.html]
 [test_sanityException2.html]
 [test_sanityParams.html]
 [test_sanityRegisteredServiceWorker.html]
@@ -35,10 +34,9 @@ skip-if = toolkit == 'android'  # bug 68
 subsuite = clipboard
 skip-if = toolkit == 'android'  # bug 688052
 [test_sanity_manifest.html]
 skip-if = toolkit == 'android' # we use the old manifest style on android
 fail-if = true
 [test_sanity_manifest_pf.html]
 skip-if = toolkit == 'android' # we use the old manifest style on android
 fail-if = true
-[test_spawn_task.html]
 [test_sanity_waitForCondition.html]

deleted file mode 100644
--- a/testing/mochitest/tests/Harness_sanity/test_add_task.html
+++ /dev/null
@@ -1,38 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<head>
-  <title>Test for mochitest add_task, found in SpawnTask.js</title>
-  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
-  <script type="text/javascript" src="/tests/SimpleTest/SpawnTask.js"></script>
-  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
-</head>
-<body>
-<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=">Mozilla Bug 1187701</a>
-<p id="display"></p>
-<div id="content" style="display: none"></div>
-<pre id="test">
-<script class="testbody" type="text/javascript">
-
-// Check that we can 'add_task' a few times and all tasks run asynchronously before test finishes.
-
-add_task(async function() {
-  var x = await Promise.resolve(1);
-  is(x, 1, "task yields Promise value as expected");
-});
-
-add_task(function* () {
-  var x = yield [Promise.resolve(1), Promise.resolve(2), Promise.resolve(3)];
-  is(x.join(""), "123", "task yields Promise value as expected");
-});
-
-add_task(function* () {
-  var x = yield (function* () {
-    return 3;
-  }());
-  is(x, 3, "task yields generator function return value as expected");
-});
-</script>
-</pre>
-</body>
-</html>
-

deleted file mode 100644
--- a/testing/mochitest/tests/Harness_sanity/test_spawn_task.html
+++ /dev/null
@@ -1,73 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<head>
-  <title>Test for mochitest SpawnTask.js sanity</title>
-  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
-  <script type="text/javascript" src="/tests/SimpleTest/SpawnTask.js"></script>
-  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
-</head>
-<body>
-<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=">Mozilla Bug 1078657</a>
-<p id="display"></p>
-<div id="content" style="display: none"></div>
-<pre id="test">
-<script class="testbody" type="text/javascript">
-
-/** Test for sanity  **/
-SimpleTest.waitForExplicitFinish();
-
-var externalGeneratorFunction = function* () {
-  return 8;
-};
-
-var nestedFunction = function* () {
-  return yield function* () {
-    return yield function* () {
-      return yield function* () {
-        return yield Promise.resolve(9);
-      }();
-    }();
-  }();
-}
-
-var variousTests = function* () {
-  var val1 = yield [Promise.resolve(1), Promise.resolve(2), Promise.resolve(3)];
-  is(val1.join(""), "123", "Array of promises -> Promise.all");
-  var val2 = yield Promise.resolve(2);
-  is(val2, 2, "Resolved promise yields value.");
-  var val3 = yield function* () { return 3; };
-  is(val3, 3, "Generator functions are spawned.");
-  //var val4 = yield function () { return 4; };
-  //is(val4, 4, "Plain functions run and return.");
-  var val5 = yield (function* () { return 5; }());
-  is(val5, 5, "Generators are spawned.");
-  try {
-    var val6 = yield Promise.reject(Error("error6"));
-    ok(false, "Shouldn't reach this line.");
-  } catch (error) {
-    is(error.message, "error6", "Rejected promise throws error.");
-  }
-  try {
-    var val7 = yield function* () { throw Error("error7"); };
-    ok(false, "Shouldn't reach this line.");
-  } catch (error) {
-    is(error.message, "error7", "Thrown error propagates.");
-  }
-  var val8 = yield externalGeneratorFunction();
-  is(val8, 8, "External generator also spawned.");
-  var val9 = yield nestedFunction();
-  is(val9, 9, "Nested generator functions work.");
-  return 10;
-};
-
-spawn_task(variousTests).then(function(result) {
-  is(result, 10, "spawn_task(...) returns promise");
-  SimpleTest.finish();
-});
-
-
-</script>
-</pre>
-</body>
-</html>
-

deleted file mode 100644
--- a/testing/mochitest/tests/SimpleTest/LICENSE_SpawnTask
+++ /dev/null
@@ -1,24 +0,0 @@
-LICENSE for SpawnTask.js (the co library):
-
-(The MIT License)
-
-Copyright (c) 2014 TJ Holowaychuk <tj@vision-media.ca>
-
-Permission is hereby granted, free of charge, to any person obtaining
-a copy of this software and associated documentation files (the
-'Software'), to deal in the Software without restriction, including
-without limitation the rights to use, copy, modify, merge, publish,
-distribute, sublicense, and/or sell copies of the Software, and to
-permit persons to whom the Software is furnished to do so, subject to
-the following conditions:
-
-The above copyright notice and this permission notice shall be
-included in all copies or substantial portions of the Software.
-
-THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,
-EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
-MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
-IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
-CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
-TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
-SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

--- a/testing/mochitest/tests/SimpleTest/SpawnTask.js
+++ b/testing/mochitest/tests/SimpleTest/SpawnTask.js
@@ -1,280 +1,39 @@
-// # SpawnTask.js
-// Directly copied from the "co" library by TJ Holowaychuk.
-// See https://github.com/tj/co/tree/4.6.0
-// For use with mochitest-plain and mochitest-chrome.
-
-// spawn_task(generatorFunction):
-// Expose only the `co` function, which is very similar to Task.spawn in Task.jsm.
-// We call this function spawn_task to make its purpose more plain, and to
-// reduce the chance of name collisions.
-var spawn_task = (function () {
-
-/**
- * slice() reference.
- */
-
-var slice = Array.prototype.slice;
-
-/**
- * Wrap the given generator `fn` into a
- * function that returns a promise.
- * This is a separate function so that
- * every `co()` call doesn't create a new,
- * unnecessary closure.
- *
- * @param {GeneratorFunction} fn
- * @return {Function}
- * @api public
- */
-
-co.wrap = function (fn) {
-  createPromise.__generatorFunction__ = fn;
-  return createPromise;
-  function createPromise() {
-    return co.call(this, fn.apply(this, arguments));
-  }
-};
-
-/**
- * Execute the generator function or a generator
- * and return a promise.
- *
- * @param {Function} fn
- * @return {Promise}
- * @api public
- */
-
-function co(gen) {
-  var ctx = this;
-  var args = slice.call(arguments, 1)
-
-  // we wrap everything in a promise to avoid promise chaining,
-  // which leads to memory leak errors.
-  // see https://github.com/tj/co/issues/180
-  return new Promise(function(resolve, reject) {
-    if (typeof gen === 'function') gen = gen.apply(ctx, args);
-    if (!gen || typeof gen.next !== 'function') return resolve(gen);
-
-    onFulfilled();
-
-    /**
-     * @param {Mixed} res
-     * @return {Promise}
-     * @api private
-     */
-
-    function onFulfilled(res) {
-      var ret;
-      try {
-        ret = gen.next(res);
-      } catch (e) {
-        return reject(e);
-      }
-      next(ret);
-    }
-
-    /**
-     * @param {Error} err
-     * @return {Promise}
-     * @api private
-     */
-
-    function onRejected(err) {
-      var ret;
-      try {
-        ret = gen.throw(err);
-      } catch (e) {
-        return reject(e);
-      }
-      next(ret);
-    }
-
-    /**
-     * Get the next value in the generator,
-     * return a promise.
-     *
-     * @param {Object} ret
-     * @return {Promise}
-     * @api private
-     */
-
-    function next(ret) {
-      if (ret.done) return resolve(ret.value);
-      var value = toPromise.call(ctx, ret.value);
-      if (value && isPromise(value)) return value.then(onFulfilled, onRejected);
-      return onRejected(new TypeError('You may only yield a function, promise, generator, array, or object, '
-        + 'but the following object was passed: "' + String(ret.value) + '"'));
-    }
-  });
-}
-
-/**
- * Convert a `yield`ed value into a promise.
- *
- * @param {Mixed} obj
- * @return {Promise}
- * @api private
- */
-
-function toPromise(obj) {
-  if (!obj) return obj;
-  if (isPromise(obj)) return obj;
-  if (isGeneratorFunction(obj) || isGenerator(obj)) return co.call(this, obj);
-  if ('function' == typeof obj) return thunkToPromise.call(this, obj);
-  if (Array.isArray(obj)) return arrayToPromise.call(this, obj);
-  if (isObject(obj)) return objectToPromise.call(this, obj);
-  return obj;
-}
-
-/**
- * Convert a thunk to a promise.
- *
- * @param {Function}
- * @return {Promise}
- * @api private
- */
-
-function thunkToPromise(fn) {
-  var ctx = this;
-  return new Promise(function (resolve, reject) {
-    fn.call(ctx, function (err, res) {
-      if (err) return reject(err);
-      if (arguments.length > 2) res = slice.call(arguments, 1);
-      resolve(res);
-    });
-  });
-}
-
-/**
- * Convert an array of "yieldables" to a promise.
- * Uses `Promise.all()` internally.
- *
- * @param {Array} obj
- * @return {Promise}
- * @api private
- */
-
-function arrayToPromise(obj) {
-  return Promise.all(obj.map(toPromise, this));
-}
-
-/**
- * Convert an object of "yieldables" to a promise.
- * Uses `Promise.all()` internally.
- *
- * @param {Object} obj
- * @return {Promise}
- * @api private
- */
-
-function objectToPromise(obj){
-  var results = new obj.constructor();
-  var keys = Object.keys(obj);
-  var promises = [];
-  for (var i = 0; i < keys.length; i++) {
-    var key = keys[i];
-    var promise = toPromise.call(this, obj[key]);
-    if (promise && isPromise(promise)) defer(promise, key);
-    else results[key] = obj[key];
-  }
-  return Promise.all(promises).then(function () {
-    return results;
-  });
-
-  function defer(promise, key) {
-    // predefine the key in the result
-    results[key] = undefined;
-    promises.push(promise.then(function (res) {
-      results[key] = res;
-    }));
-  }
-}
-
-/**
- * Check if `obj` is a promise.
- *
- * @param {Object} obj
- * @return {Boolean}
- * @api private
- */
-
-function isPromise(obj) {
-  return 'function' == typeof obj.then;
-}
-
-/**
- * Check if `obj` is a generator.
- *
- * @param {Mixed} obj
- * @return {Boolean}
- * @api private
- */
-
-function isGenerator(obj) {
-  return 'function' == typeof obj.next && 'function' == typeof obj.throw;
-}
-
-/**
- * Check if `obj` is a generator function.
- *
- * @param {Mixed} obj
- * @return {Boolean}
- * @api private
- */
-function isGeneratorFunction(obj) {
-  var constructor = obj.constructor;
-  if (!constructor) return false;
-  if ('GeneratorFunction' === constructor.name || 'GeneratorFunction' === constructor.displayName) return true;
-  return isGenerator(constructor.prototype);
-}
-
-/**
- * Check for plain object.
- *
- * @param {Mixed} val
- * @return {Boolean}
- * @api private
- */
-
-function isObject(val) {
-  return Object == val.constructor;
-}
-
-return co;
-})();
-
 // add_task(generatorFunction):
 // Call `add_task(generatorFunction)` for each separate
 // asynchronous task in a mochitest. Tasks are run consecutively.
 // Before the first task, `SimpleTest.waitForExplicitFinish()`
 // will be called automatically, and after the last task,
 // `SimpleTest.finish()` will be called.
 var add_task = (function () {
   // The list of tasks to run.
   var task_list = [];
   var run_only_this_task = null;
 
+  function isGenerator(value) {
+    return value && typeof value === "object" && typeof value.next === "function";
+  }
+
   // The "add_task" function
   return function (generatorFunction) {
     if (task_list.length === 0) {
       // This is the first time add_task has been called.
       // First, confirm that SimpleTest is available.
       if (!SimpleTest) {
         throw new Error("SimpleTest not available.");
       }
       // Don't stop tests until asynchronous tasks are finished.
       SimpleTest.waitForExplicitFinish();
       // Because the client is using add_task for this set of tests,
       // we need to spawn a "master task" that calls each task in succesion.
       // Use setTimeout to ensure the master task runs after the client
       // script finishes.
       setTimeout(function () {
-        spawn_task(function* () {
+        (async () => {
           // Allow for a task to be skipped; we need only use the structured logger
           // for this, whilst deactivating log buffering to ensure that messages
           // are always printed to stdout.
           function skipTask(name) {
             let logger = parentRunner && parentRunner.structuredLogger;
             if (!logger) {
               info("SpawnTask.js | Skipping test " + name);
               return;
@@ -290,30 +49,33 @@ var add_task = (function () {
           try {
             for (var task of task_list) {
               var name = task.name || "";
               if (task.__skipMe || (run_only_this_task && task != run_only_this_task)) {
                 skipTask(name);
                 continue;
               }
               info("SpawnTask.js | Entering test " + name);
-              yield task();
+              let result = await task();
+              if (isGenerator(result)) {
+                ok(false, "Task returned a generator");
+              }
               info("SpawnTask.js | Leaving test " + name);
             }
           } catch (ex) {
             try {
               ok(false, "" + ex, "Should not throw any errors", ex.stack);
             } catch (ex2) {
               ok(false, "(The exception cannot be converted to string.)",
                  "Should not throw any errors", ex.stack);
             }
           }
           // All tasks are finished.
           SimpleTest.finish();
-        });
+        })();
       });
     }
     generatorFunction.skip = () => generatorFunction.__skipMe = true;
     generatorFunction.only = () => run_only_this_task = generatorFunction;
     // Add the task to the list of tasks to run after
     // the main thread is finished.
     task_list.push(generatorFunction);
     return generatorFunction;

--- a/toolkit/components/contentprefs/tests/mochitest/test_remoteContentPrefs.html
+++ b/toolkit/components/contentprefs/tests/mochitest/test_remoteContentPrefs.html
@@ -319,18 +319,18 @@
       let deferred = Defer();
       iframe.addEventListener("mozbrowserloadend", function() {
         info("Got iframe load event.");
         let mm = SpecialPowers.getBrowserFrameMessageManager(iframe);
         mm.loadFrameScript("data:,(" + childFrameScript.toString() + ")(" + isPrivate + ");",
                            false);
 
         // Chain testStructure to runTests's promise.
-        spawn_task(testStructure(mm, isPrivate)).then(deferred.resolve)
-                                                .catch((e) => { info(`caught failing test ${e}`); });
+        testStructure(mm, isPrivate).then(deferred.resolve)
+                                    .catch((e) => { info(`caught failing test ${e}`); });
       });
 
       document.body.appendChild(iframe);
       return deferred.promise;
     }
 
     function runTests() {
       info("Browser prefs set.");

--- a/toolkit/components/places/tests/chrome/browser_disableglobalhistory.xul
+++ b/toolkit/components/places/tests/chrome/browser_disableglobalhistory.xul
@@ -1,14 +1,13 @@
 <?xml version="1.0"?>
 <?xml-stylesheet href="chrome://global/skin" type="text/css"?>
 
 <window title="Test disableglobalhistory attribute on remote browsers"
-  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
-  onload="run_test();">
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
   <script type="application/javascript"
           src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
   <script type="application/javascript"
           src="chrome://mochikit/content/tests/SimpleTest/SpawnTask.js"></script>
 
   <browser id="inprocess_disabled" src="about:blank" type="content" disableglobalhistory="true" />
   <browser id="inprocess_enabled" src="about:blank" type="content" />
 
@@ -23,21 +22,20 @@
   function expectUseGlobalHistory(id, expected) {
     let browser = document.getElementById(id);
     return ContentTask.spawn(browser, {id, expected}, function({id, expected}) {
       Assert.equal(docShell.useGlobalHistory, expected,
                    "Got the right useGlobalHistory state in the docShell of " + id);
     });
   }
 
-  function run_test() {
-    spawn_task(function*() {
-      yield expectUseGlobalHistory("inprocess_disabled", false);
-      yield expectUseGlobalHistory("inprocess_enabled", true);
+  add_task(async function() {
+    await expectUseGlobalHistory("inprocess_disabled", false);
+    await expectUseGlobalHistory("inprocess_enabled", true);
 
-      yield expectUseGlobalHistory("remote_disabled", false);
-      yield expectUseGlobalHistory("remote_enabled", true);
-      window.opener.done();
-    });
-  };
+    await expectUseGlobalHistory("remote_disabled", false);
+    await expectUseGlobalHistory("remote_enabled", true);
+    window.opener.done();
+    ok(true);
+  });
 
   </script>
 </window>

--- a/toolkit/components/places/tests/chrome/test_browser_disableglobalhistory.xul
+++ b/toolkit/components/places/tests/chrome/test_browser_disableglobalhistory.xul
@@ -14,13 +14,12 @@
 
   <script type="text/javascript">
   SimpleTest.waitForExplicitFinish();
 
   let w = window.open('browser_disableglobalhistory.xul', '_blank', 'chrome,resizable=yes,width=400,height=600');
 
   function done() {
     w.close();
-    SimpleTest.finish();
   }
   </script>
 
-</window>
\ No newline at end of file
+</window>

--- a/toolkit/components/satchel/test/test_datalist_with_caching.html
+++ b/toolkit/components/satchel/test/test_datalist_with_caching.html
@@ -36,17 +36,17 @@ Form History test: form field autocomple
 
 let input = $_(1, "field1");
 
 function setupFormHistory(aCallback) {
   updateFormHistory([
     { op: "remove" },
     { op: "add", fieldname: "field1", value: "Sec" },
   ], () => {
-    spawn_task(aCallback);
+    aCallback();
   });
 }
 
 function setForm(value) {
   input.value = value;
   input.focus();
 }