diff --git a/devtools/server/tests/unit/test_objectgrips-12.js b/devtools/server/tests/unit/test_objectgrips-12.js
index e0c81523fb7..307ce54ce28 100644
--- a/devtools/server/tests/unit/test_objectgrips-12.js
+++ b/devtools/server/tests/unit/test_objectgrips-12.js
@@ -3,6 +3,8 @@
 
 // Test getDisplayString.
 
+Cu.import("resource://testing-common/PromiseTestUtils.jsm", this);
+
 var gDebuggee;
 var gClient;
 var gThreadClient;
@@ -125,6 +127,7 @@ function test_display_string()
       output: "Promise (fulfilled: 5)"
     },
     {
+      // This rejection is left uncaught, see expectUncaughtRejection below.
       input: "Promise.reject(new Error())",
       output: "Promise (rejected: Error)"
     },
@@ -134,6 +137,8 @@ function test_display_string()
     }
   ];
 
+  PromiseTestUtils.expectUncaughtRejection(/Error/);
+
   gThreadClient.addOneTimeListener("paused", function(aEvent, aPacket) {
     const args = aPacket.frame.arguments;
 
diff --git a/devtools/server/tests/unit/test_promises_actor_onpromisesettled.js b/devtools/server/tests/unit/test_promises_actor_onpromisesettled.js
index 970e171a696..77012b1da4b 100644
--- a/devtools/server/tests/unit/test_promises_actor_onpromisesettled.js
+++ b/devtools/server/tests/unit/test_promises_actor_onpromisesettled.js
@@ -8,6 +8,8 @@
 
 "use strict";
 
+Cu.import("resource://testing-common/PromiseTestUtils.jsm", this);
+
 const { PromisesFront } = require("devtools/server/actors/promises");
 
 var events = require("sdk/event/core");
@@ -52,6 +54,7 @@ function* testPromisesSettled(client, form, makeResolvePromise,
   let foundResolvedPromise = yield onPromiseSettled;
   ok(foundResolvedPromise, "Found our resolved promise");
 
+  PromiseTestUtils.expectUncaughtRejection(r => r.message == resolution);
   onPromiseSettled = oncePromiseSettled(front, resolution, false, true);
   let rejectedPromise = makeRejectPromise(resolution);
   let foundRejectedPromise = yield onPromiseSettled;
diff --git a/devtools/shared/acorn/tests/unit/head_acorn.js b/devtools/shared/acorn/tests/unit/head_acorn.js
index 5cddb4355a9..3e06ca3c1a9 100644
--- a/devtools/shared/acorn/tests/unit/head_acorn.js
+++ b/devtools/shared/acorn/tests/unit/head_acorn.js
@@ -62,6 +62,11 @@ var listener = {
       }
     }
 
+    // Ignored until they are fixed in bug 1242968.
+    if (string.includes("JavaScript Warning")) {
+      return;
+    }
+
     do_throw("head_acorn.js got console message: " + string + "\n");
   }
 };
diff --git a/devtools/shared/pretty-fast/tests/unit/head_pretty-fast.js b/devtools/shared/pretty-fast/tests/unit/head_pretty-fast.js
index af999f4deff..abde4b197e5 100644
--- a/devtools/shared/pretty-fast/tests/unit/head_pretty-fast.js
+++ b/devtools/shared/pretty-fast/tests/unit/head_pretty-fast.js
@@ -35,6 +35,11 @@ var listener = {
       }
     }
 
+    // Ignored until they are fixed in bug 1242968.
+    if (string.includes("JavaScript Warning")) {
+      return;
+    }
+
     do_throw("head_pretty-fast.js got console message: " + string + "\n");
   }
 };
diff --git a/dom/promise/tests/unit/test_monitor_uncaught.js b/dom/promise/tests/unit/test_monitor_uncaught.js
index f42bef52596..7dd80d212c1 100644
--- a/dom/promise/tests/unit/test_monitor_uncaught.js
+++ b/dom/promise/tests/unit/test_monitor_uncaught.js
@@ -7,6 +7,10 @@
 var { utils: Cu } = Components;
 
 Cu.import("resource://gre/modules/Timer.jsm", this);
+Cu.import("resource://testing-common/PromiseTestUtils.jsm", this);
+
+// Prevent test failures due to the unhandled rejections in this test file.
+PromiseTestUtils.disableUncaughtRejectionObserverForSelfTest();
 
 add_task(function* test_globals() {
   Assert.equal(Promise.defer || undefined, undefined, "We are testing DOM Promise.");
diff --git a/dom/push/PushService.jsm b/dom/push/PushService.jsm
index 486c7d7be98..ddb7b2491d9 100644
--- a/dom/push/PushService.jsm
+++ b/dom/push/PushService.jsm
@@ -458,7 +458,7 @@ this.PushService = {
         // Before completing the activation check prefs. This will first check
         // connection.enabled pref and then check offline state.
         this._changeStateConnectionEnabledEvent(prefs.get("connection.enabled"));
-      });
+      }).catch(Cu.reportError);
 
     } else {
       // This is only used for testing. Different tests require connecting to
diff --git a/dom/push/PushServiceHttp2.jsm b/dom/push/PushServiceHttp2.jsm
index efeedc80c82..7e2048c53f5 100644
--- a/dom/push/PushServiceHttp2.jsm
+++ b/dom/push/PushServiceHttp2.jsm
@@ -726,12 +726,15 @@ this.PushServiceHttp2 = {
       .then(record => this._subscribeResource(record)
         .then(recordNew => {
           if (this._mainPushService) {
-            this._mainPushService.updateRegistrationAndNotifyApp(aSubscriptionUri,
-                                                                 recordNew);
+            this._mainPushService
+                .updateRegistrationAndNotifyApp(aSubscriptionUri, recordNew)
+                .catch(Cu.reportError);
           }
         }, error => {
           if (this._mainPushService) {
-            this._mainPushService.dropRegistrationAndNotifyApp(aSubscriptionUri);
+            this._mainPushService
+                .dropRegistrationAndNotifyApp(aSubscriptionUri)
+                .catch(Cu.reportError);
           }
         })
       );
diff --git a/dom/push/test/xpcshell/test_registration_success_http2.js b/dom/push/test/xpcshell/test_registration_success_http2.js
index e7d3c897f3a..f955ba2688e 100644
--- a/dom/push/test/xpcshell/test_registration_success_http2.js
+++ b/dom/push/test/xpcshell/test_registration_success_http2.js
@@ -4,6 +4,15 @@
 'use strict';
 
 Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://testing-common/PromiseTestUtils.jsm");
+
+///////////////////
+//
+// Whitelisting this test.
+// As part of bug 1077403, the leaking uncaught rejection should be fixed.
+//
+// Instances of the rejection "record is undefined" may or may not appear.
+PromiseTestUtils.thisTestLeaksUncaughtRejectionsAndShouldBeFixed();
 
 const {PushDB, PushService, PushServiceHttp2} = serviceExports;
 
diff --git a/dom/push/test/xpcshell/test_unregister_success_http2.js b/dom/push/test/xpcshell/test_unregister_success_http2.js
index b6c77ca3d6d..c7c63f82933 100644
--- a/dom/push/test/xpcshell/test_unregister_success_http2.js
+++ b/dom/push/test/xpcshell/test_unregister_success_http2.js
@@ -4,6 +4,15 @@
 'use strict';
 
 Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://testing-common/PromiseTestUtils.jsm");
+
+///////////////////
+//
+// Whitelisting this test.
+// As part of bug 1077403, the leaking uncaught rejection should be fixed.
+//
+// Instances of the rejection "record is undefined" may or may not appear.
+PromiseTestUtils.thisTestLeaksUncaughtRejectionsAndShouldBeFixed();
 
 const {PushDB, PushService, PushServiceHttp2} = serviceExports;
 
diff --git a/testing/xpcshell/head.js b/testing/xpcshell/head.js
index da78df981e2..4df2a068d70 100644
--- a/testing/xpcshell/head.js
+++ b/testing/xpcshell/head.js
@@ -22,6 +22,7 @@ var _profileInitialized = false;
 _register_modules_protocol_handler();
 
 var _Promise = Components.utils.import("resource://gre/modules/Promise.jsm", {}).Promise;
+var _PromiseTestUtils = Components.utils.import("resource://testing-common/PromiseTestUtils.jsm", {}).PromiseTestUtils;
 
 // Support a common assertion library, Assert.jsm.
 var AssertCls = Components.utils.import("resource://testing-common/Assert.jsm", null).Assert;
@@ -213,7 +214,6 @@ function _do_main() {
 
 function _do_quit() {
   _testLogger.info("exiting test");
-  _Promise.Debugging.flushUncaughtErrors();
   _quit = true;
 }
 
@@ -499,16 +499,8 @@ function _execute_test() {
   // Call do_get_idle() to restore the factory and get the service.
   _fakeIdleService.activate();
 
-  _Promise.Debugging.clearUncaughtErrorObservers();
-  _Promise.Debugging.addUncaughtErrorObserver(function observer({message, date, fileName, stack, lineNumber}) {
-    let text = " A promise chain failed to handle a rejection: " +
-        message + " - rejection date: " + date;
-    _testLogger.error(text,
-                      {
-                        stack: _format_stack(stack),
-                        source_file: fileName
-                      });
-  });
+  _PromiseTestUtils.init();
+  _PromiseTestUtils.Assert = Assert;
 
   // _HEAD_FILES is dynamically defined by <runxpcshelltests.py>.
   _load_files(_HEAD_FILES);
@@ -539,6 +531,7 @@ function _execute_test() {
     }
     do_test_finished("MAIN run_test");
     _do_main();
+    _PromiseTestUtils.assertNoUncaughtRejections();
   } catch (e) {
     _passed = false;
     // do_check failures are already logged and set _quit to true and throw
@@ -587,6 +580,9 @@ function _execute_test() {
                       });
   };
 
+  let thr = Components.classes["@mozilla.org/thread-manager;1"]
+              .getService().currentThread;
+
   let func;
   while ((func = _cleanupFunctions.pop())) {
     let result;
@@ -602,8 +598,6 @@ function _execute_test() {
       let complete = false;
       let promise = result.then(null, reportCleanupError);
       promise = promise.then(() => complete = true);
-      let thr = Components.classes["@mozilla.org/thread-manager;1"]
-                  .getService().currentThread;
       while (!complete) {
         thr.processNextEvent(true);
       }
@@ -613,8 +607,14 @@ function _execute_test() {
   // Restore idle service to avoid leaks.
   _fakeIdleService.deactivate();
 
-  if (!_passed)
-    return;
+  try {
+    _PromiseTestUtils.ensureDOMPromiseRejectionsProcessed();
+    _PromiseTestUtils.assertNoUncaughtRejections();
+    _PromiseTestUtils.assertNoMoreExpectedRejections();
+  } finally {
+    // It's important to terminate the module to avoid crashes on shutdown.
+    _PromiseTestUtils.uninit();
+  }
 }
 
 /**
@@ -1516,8 +1516,8 @@ function run_next_test()
   function _run_next_test()
   {
     if (_gTestIndex < _gTests.length) {
-      // Flush uncaught errors as early and often as possible.
-      _Promise.Debugging.flushUncaughtErrors();
+      // Check for uncaught rejections as early and often as possible.
+      _PromiseTestUtils.assertNoUncaughtRejections();
       let _properties;
       [_properties, _gRunningTest,] = _gTests[_gTestIndex++];
       if (typeof(_properties.skip_if) == "function" && _properties.skip_if()) {
@@ -1538,10 +1538,18 @@ function run_next_test()
 
       if (_properties._isTask) {
         _gTaskRunning = true;
-        _Task.spawn(_gRunningTest).then(
-          () => { _gTaskRunning = false; run_next_test(); },
-          (ex) => { _gTaskRunning = false; do_report_unexpected_exception(ex); }
-        );
+        _Task.spawn(_gRunningTest).then(() => {
+          _gTaskRunning = false;
+          run_next_test();
+        }, ex => {
+          _gTaskRunning = false;
+          try {
+            do_report_unexpected_exception(ex);
+          } catch (ex) {
+            // The above throws NS_ERROR_ABORT and we don't want this to show up
+            // as an unhandled rejection later.
+          }
+        });
       } else {
         // Exceptions do not kill asynchronous tests, so they'll time out.
         try {
diff --git a/testing/xpcshell/selftest.py b/testing/xpcshell/selftest.py
index 854d3268975..e389e4b4713 100644
--- a/testing/xpcshell/selftest.py
+++ b/testing/xpcshell/selftest.py
@@ -35,6 +35,23 @@ TEST_FAIL_STRING = "TEST-UNEXPECTED-FAIL"
 SIMPLE_PASSING_TEST = "function run_test() { do_check_true(true); }"
 SIMPLE_FAILING_TEST = "function run_test() { do_check_true(false); }"
 
+SIMPLE_UNCAUGHT_REJECTION_TEST = '''
+function run_test() {
+  Promise.reject(new Error("Test rejection."));
+  do_check_true(true);
+}
+'''
+
+SIMPLE_UNCAUGHT_REJECTION_JSM_TEST = '''
+Components.utils.import("resource://gre/modules/Promise.jsm");
+
+Promise.reject(new Error("Test rejection."));
+
+function run_test() {
+  do_check_true(true);
+}
+'''
+
 ADD_TEST_SIMPLE = '''
 function run_test() { run_next_test(); }
 
@@ -53,6 +70,26 @@ add_test(function test_failing() {
 });
 '''
 
+ADD_TEST_UNCAUGHT_REJECTION = '''
+function run_test() { run_next_test(); }
+
+add_test(function test_uncaught_rejection() {
+  Promise.reject(new Error("Test rejection."));
+  run_next_test();
+});
+'''
+
+ADD_TEST_UNCAUGHT_REJECTION_JSM = '''
+Components.utils.import("resource://gre/modules/Promise.jsm");
+
+function run_test() { run_next_test(); }
+
+add_test(function test_uncaught_rejection() {
+  Promise.reject(new Error("Test rejection."));
+  run_next_test();
+});
+'''
+
 CHILD_TEST_PASSING = '''
 function run_test () { run_next_test(); }
 
@@ -424,6 +461,7 @@ tail =
                                           shuffle=shuffle,
                                           verbose=verbose,
                                           sequential=True,
+                                          testingModulesDir=os.path.join(objdir, '_tests', 'modules'),
                                           utility_path=self.utility_path),
                           msg="""Tests should have %s, log:
 ========
@@ -802,6 +840,30 @@ add_test({
         self.assertInLog(TEST_FAIL_STRING)
         self.assertNotInLog(TEST_PASS_STRING)
 
+    def testUncaughtRejection(self):
+        """
+        Ensure a simple test with an uncaught rejection is reported.
+        """
+        self.writeFile("test_simple_uncaught_rejection.js", SIMPLE_UNCAUGHT_REJECTION_TEST)
+        self.writeManifest(["test_simple_uncaught_rejection.js"])
+
+        self.assertTestResult(False)
+        self.assertEquals(1, self.x.testCount)
+        self.assertEquals(0, self.x.passCount)
+        self.assertEquals(1, self.x.failCount)
+
+    def testUncaughtRejectionJSM(self):
+        """
+        Ensure a simple test with an uncaught rejection from Promise.jsm is reported.
+        """
+        self.writeFile("test_simple_uncaught_rejection_jsm.js", SIMPLE_UNCAUGHT_REJECTION_JSM_TEST)
+        self.writeManifest(["test_simple_uncaught_rejection_jsm.js"])
+
+        self.assertTestResult(False)
+        self.assertEquals(1, self.x.testCount)
+        self.assertEquals(0, self.x.passCount)
+        self.assertEquals(1, self.x.failCount)
+
     def testAddTestSimple(self):
         """
         Ensure simple add_test() works.
@@ -839,6 +901,30 @@ add_test({
         self.assertEquals(0, self.x.passCount)
         self.assertEquals(1, self.x.failCount)
 
+    def testAddTestUncaughtRejection(self):
+        """
+        Ensure add_test() with an uncaught rejection is reported.
+        """
+        self.writeFile("test_add_test_uncaught_rejection.js", ADD_TEST_UNCAUGHT_REJECTION)
+        self.writeManifest(["test_add_test_uncaught_rejection.js"])
+
+        self.assertTestResult(False)
+        self.assertEquals(1, self.x.testCount)
+        self.assertEquals(0, self.x.passCount)
+        self.assertEquals(1, self.x.failCount)
+
+    def testAddTestUncaughtRejectionJSM(self):
+        """
+        Ensure add_test() with an uncaught rejection from Promise.jsm is reported.
+        """
+        self.writeFile("test_add_test_uncaught_rejection_jsm.js", ADD_TEST_UNCAUGHT_REJECTION_JSM)
+        self.writeManifest(["test_add_test_uncaught_rejection_jsm.js"])
+
+        self.assertTestResult(False)
+        self.assertEquals(1, self.x.testCount)
+        self.assertEquals(0, self.x.passCount)
+        self.assertEquals(1, self.x.failCount)
+
     def testAddTaskTestSingle(self):
         """
         Ensure add_test_task() with a single passing test works.
diff --git a/toolkit/components/jsdownloads/src/DownloadCore.jsm b/toolkit/components/jsdownloads/src/DownloadCore.jsm
index 01d5eabe616..7b2a508165a 100644
--- a/toolkit/components/jsdownloads/src/DownloadCore.jsm
+++ b/toolkit/components/jsdownloads/src/DownloadCore.jsm
@@ -2101,6 +2101,9 @@ this.DownloadCopySaver.prototype = {
           // In case an error occurs while setting up the chain of objects for
           // the download, ensure that we release the resources of the saver.
           backgroundFileSaver.finish(Cr.NS_ERROR_FAILURE);
+          // Since we're not going to handle deferSaveComplete.promise below,
+          // we need to make sure that the rejection is handled.
+          deferSaveComplete.promise.catch(() => {});
           throw ex;
         }
 
diff --git a/toolkit/components/jsdownloads/src/DownloadImport.jsm b/toolkit/components/jsdownloads/src/DownloadImport.jsm
index ba85761c0ea..6c175c0239c 100644
--- a/toolkit/components/jsdownloads/src/DownloadImport.jsm
+++ b/toolkit/components/jsdownloads/src/DownloadImport.jsm
@@ -174,7 +174,7 @@ this.DownloadImport.prototype = {
             yield this.list.add(download);
 
             if (resumeDownload) {
-              download.start();
+              download.start().catch(() => {});
             } else {
               yield download.refresh();
             }
diff --git a/toolkit/components/jsdownloads/src/DownloadIntegration.jsm b/toolkit/components/jsdownloads/src/DownloadIntegration.jsm
index aeceb5f2b76..839035b9d44 100644
--- a/toolkit/components/jsdownloads/src/DownloadIntegration.jsm
+++ b/toolkit/components/jsdownloads/src/DownloadIntegration.jsm
@@ -1080,7 +1080,7 @@ this.DownloadObserver = {
     this._wakeTimer = null;
 
     for (let download of this._canceledOfflineDownloads) {
-      download.start();
+      download.start().catch(() => {});
     }
   },
 
diff --git a/toolkit/components/jsdownloads/src/DownloadLegacy.js b/toolkit/components/jsdownloads/src/DownloadLegacy.js
index 48218d87ad9..c4b63808551 100644
--- a/toolkit/components/jsdownloads/src/DownloadLegacy.js
+++ b/toolkit/components/jsdownloads/src/DownloadLegacy.js
@@ -243,7 +243,7 @@ DownloadLegacyTransfer.prototype = {
       }
 
       // Start the download before allowing it to be controlled.  Ignore errors.
-      aDownload.start().then(null, () => {});
+      aDownload.start().catch(() => {});
 
       // Start processing all the other events received through nsITransfer.
       this._deferDownload.resolve(aDownload);
diff --git a/toolkit/components/jsdownloads/src/DownloadStore.jsm b/toolkit/components/jsdownloads/src/DownloadStore.jsm
index 8ad7e720d63..83bc2824fbb 100644
--- a/toolkit/components/jsdownloads/src/DownloadStore.jsm
+++ b/toolkit/components/jsdownloads/src/DownloadStore.jsm
@@ -124,8 +124,8 @@ this.DownloadStore.prototype = {
           try {
             if (!download.succeeded && !download.canceled && !download.error) {
               // Try to restart the download if it was in progress during the
-              // previous session.
-              download.start();
+              // previous session.  Ignore errors.
+              download.start().catch(() => {});
             } else {
               // If the download was not in progress, try to update the current
               // progress from disk.  This is relevant in case we retained
diff --git a/toolkit/components/jsdownloads/test/browser/browser_DownloadPDFSaver.js b/toolkit/components/jsdownloads/test/browser/browser_DownloadPDFSaver.js
index 63e1174656b..e08e500f78c 100644
--- a/toolkit/components/jsdownloads/test/browser/browser_DownloadPDFSaver.js
+++ b/toolkit/components/jsdownloads/test/browser/browser_DownloadPDFSaver.js
@@ -88,7 +88,7 @@ add_task(function* test_cancel_pdf_download() {
   });
 
   yield test_download_windowRef(tab, download);
-  download.start();
+  download.start().catch(() => {});
 
   // Immediately cancel the download to test that it is erased correctly.
   yield download.cancel();
diff --git a/toolkit/components/jsdownloads/test/unit/common_test_Download.js b/toolkit/components/jsdownloads/test/unit/common_test_Download.js
index e353452f7d6..8bedc32a627 100644
--- a/toolkit/components/jsdownloads/test/unit/common_test_Download.js
+++ b/toolkit/components/jsdownloads/test/unit/common_test_Download.js
@@ -32,7 +32,7 @@ function promiseStartDownload(aSourceUrl) {
   }
 
   return promiseNewDownload(aSourceUrl).then(download => {
-    download.start();
+    download.start().catch(() => {});
     return download;
   });
 }
@@ -64,7 +64,7 @@ function promiseStartDownload_tryToKeepPartialData() {
                   partFilePath: targetFilePath + ".part" },
       });
       download.tryToKeepPartialData = true;
-      download.start();
+      download.start().catch(() => {});
     } else {
       // Start a download using nsIExternalHelperAppService, that is configured
       // to keep partially downloaded data by default.
@@ -435,7 +435,7 @@ add_task(function* test_empty_progress_tryToKeepPartialData()
                 partFilePath: targetFilePath + ".part" },
     });
     download.tryToKeepPartialData = true;
-    download.start();
+    download.start().catch(() => {});
   } else {
     // Start a download using nsIExternalHelperAppService, that is configured
     // to keep partially downloaded data by default.
@@ -491,7 +491,7 @@ add_task(function* test_empty_noprogress()
       }
     };
 
-    download.start();
+    download.start().catch(() => {});
   } else {
     // When testing DownloadLegacySaver, the download is already started when it
     // is created, and it may have already made all needed property change
@@ -856,7 +856,7 @@ add_task(function* test_cancel_midway_restart_tryToKeepPartialData_false()
 
   // Restart the download from the beginning.
   mustInterruptResponses();
-  download.start();
+  download.start().catch(() => {});
 
   yield promiseDownloadMidway(download);
   yield promisePartFileReady(download);
@@ -1143,7 +1143,7 @@ add_task(function* test_whenSucceeded_after_restart()
     // we can verify getting a reference before the first download attempt.
     download = yield promiseNewDownload(httpUrl("interruptible.txt"));
     promiseSucceeded = download.whenSucceeded();
-    download.start();
+    download.start().catch(() => {});
   } else {
     // When testing DownloadLegacySaver, the download is already started when it
     // is created, thus we cannot get the reference before the first attempt.
@@ -1156,7 +1156,7 @@ add_task(function* test_whenSucceeded_after_restart()
 
   // The second request is allowed to complete.
   continueResponses();
-  download.start();
+  download.start().catch(() => {});
 
   // Wait for the download to finish by waiting on the whenSucceeded promise.
   yield promiseSucceeded;
@@ -1343,7 +1343,7 @@ add_task(function* test_error_restart()
         source: httpUrl("source.txt"),
         target: targetFile,
       });
-      download.start();
+      download.start().catch(() => {});
     } else {
       download = yield promiseStartLegacyDownload(null,
                                                   { targetFile: targetFile });
@@ -2186,7 +2186,7 @@ add_task(function* test_platform_integration()
         source: httpUrl("source.txt"),
         target: targetFile,
       });
-      download.start();
+      download.start().catch(() => {});
     }
 
     // Wait for the whenSucceeded promise to be resolved first.
diff --git a/toolkit/components/jsdownloads/test/unit/test_DownloadIntegration.js b/toolkit/components/jsdownloads/test/unit/test_DownloadIntegration.js
index 316407a81ee..9839cd9da9f 100644
--- a/toolkit/components/jsdownloads/test/unit/test_DownloadIntegration.js
+++ b/toolkit/components/jsdownloads/test/unit/test_DownloadIntegration.js
@@ -215,7 +215,7 @@ add_task(function* test_notifications()
     let download3 = yield promiseNewDownload(httpUrl("interruptible.txt"));
     let promiseAttempt1 = download1.start();
     let promiseAttempt2 = download2.start();
-    download3.start();
+    download3.start().catch(() => {});
 
     // Add downloads to list.
     yield list.add(download1);
@@ -250,8 +250,8 @@ add_task(function* test_no_notifications()
     let list = yield promiseNewList(isPrivate);
     let download1 = yield promiseNewDownload(httpUrl("interruptible.txt"));
     let download2 = yield promiseNewDownload(httpUrl("interruptible.txt"));
-    download1.start();
-    download2.start();
+    download1.start().catch(() => {});
+    download2.start().catch(() => {});
 
     // Add downloads to list.
     yield list.add(download1);
@@ -316,7 +316,7 @@ add_task(function* test_suspend_resume()
   {
     return Task.spawn(function* () {
       let download = yield promiseNewDownload(httpUrl("interruptible.txt"));
-      download.start();
+      download.start().catch(() => {});
       list.add(download);
       return download;
     });
diff --git a/toolkit/components/jsdownloads/test/unit/test_DownloadList.js b/toolkit/components/jsdownloads/test/unit/test_DownloadList.js
index dfb97b6f45b..93650169124 100644
--- a/toolkit/components/jsdownloads/test/unit/test_DownloadList.js
+++ b/toolkit/components/jsdownloads/test/unit/test_DownloadList.js
@@ -348,7 +348,7 @@ add_task(function* test_history_expiration()
 
   // Work with one finished download and one canceled download.
   yield downloadOne.start();
-  downloadTwo.start();
+  downloadTwo.start().catch(() => {});
   yield downloadTwo.cancel();
 
   // We must replace the visits added while executing the downloads with visits
@@ -471,7 +471,7 @@ add_task(function* test_DownloadSummary()
   // Add a public download that has been canceled midway.
   let canceledPublicDownload =
       yield promiseNewDownload(httpUrl("interruptible.txt"));
-  canceledPublicDownload.start();
+  canceledPublicDownload.start().catch(() => {});
   yield promiseDownloadMidway(canceledPublicDownload);
   yield canceledPublicDownload.cancel();
   yield publicList.add(canceledPublicDownload);
@@ -479,7 +479,7 @@ add_task(function* test_DownloadSummary()
   // Add a public download that is in progress.
   let inProgressPublicDownload =
       yield promiseNewDownload(httpUrl("interruptible.txt"));
-  inProgressPublicDownload.start();
+  inProgressPublicDownload.start().catch(() => {});
   yield promiseDownloadMidway(inProgressPublicDownload);
   yield publicList.add(inProgressPublicDownload);
 
@@ -488,7 +488,7 @@ add_task(function* test_DownloadSummary()
     source: { url: httpUrl("interruptible.txt"), isPrivate: true },
     target: getTempFile(TEST_TARGET_FILE_NAME).path,
   });
-  inProgressPrivateDownload.start();
+  inProgressPrivateDownload.start().catch(() => {});
   yield promiseDownloadMidway(inProgressPrivateDownload);
   yield privateList.add(inProgressPrivateDownload);
 
diff --git a/toolkit/components/search/nsSearchService.js b/toolkit/components/search/nsSearchService.js
index 0b50401b4db..3e10ac69e59 100644
--- a/toolkit/components/search/nsSearchService.js
+++ b/toolkit/components/search/nsSearchService.js
@@ -4453,6 +4453,13 @@ SearchService.prototype = {
   },
 
   _addObservers: function SRCH_SVC_addObservers() {
+    if (this._observersAdded) {
+      // There might be a race between synchronous and asynchronous
+      // initialization for which we try to register the observers twice.
+      return;
+    }
+    this._observersAdded = true;
+
     Services.obs.addObserver(this, SEARCH_ENGINE_TOPIC, false);
     Services.obs.addObserver(this, QUIT_APPLICATION_TOPIC, false);
 
@@ -4495,6 +4502,7 @@ SearchService.prototype = {
       () => shutdownState
     );
   },
+  _observersAdded: false,
 
   _removeObservers: function SRCH_SVC_removeObservers() {
     Services.obs.removeObserver(this, SEARCH_ENGINE_TOPIC);
diff --git a/toolkit/components/search/tests/xpcshell/test_hidden.js b/toolkit/components/search/tests/xpcshell/test_hidden.js
index f49344cd8f7..0079d05a1a8 100644
--- a/toolkit/components/search/tests/xpcshell/test_hidden.js
+++ b/toolkit/components/search/tests/xpcshell/test_hidden.js
@@ -47,10 +47,6 @@ add_task(function* async_init() {
 add_task(function* sync_init() {
   let reInitPromise = asyncReInit();
   // Synchronously check the current default engine, to force a sync init.
-  // XXX For some reason forcing a sync init while already asynchronously
-  // reinitializing causes a shutdown warning related to engineMetadataService's
-  // finalize method having already been called. Seems harmless for the purpose
-  // of this test.
   do_check_false(Services.search.isInitialized);
   do_check_eq(Services.search.currentEngine.name, "hidden");
   do_check_true(Services.search.isInitialized);
diff --git a/toolkit/modules/moz.build b/toolkit/modules/moz.build
index 2683b2bc6b7..27dffb4603b 100644
--- a/toolkit/modules/moz.build
+++ b/toolkit/modules/moz.build
@@ -9,6 +9,10 @@ BROWSER_CHROME_MANIFESTS += ['tests/browser/browser.ini']
 MOCHITEST_MANIFESTS += ['tests/mochitest/mochitest.ini']
 MOCHITEST_CHROME_MANIFESTS += ['tests/chrome/chrome.ini']
 
+TESTING_JS_MODULES += [
+    'tests/PromiseTestUtils.jsm',
+]
+
 SPHINX_TREES['toolkit_modules'] = 'docs'
 
 EXTRA_JS_MODULES += [
diff --git a/toolkit/modules/tests/PromiseTestUtils.jsm b/toolkit/modules/tests/PromiseTestUtils.jsm
new file mode 100644
index 00000000000..d60b785a580
--- /dev/null
+++ b/toolkit/modules/tests/PromiseTestUtils.jsm
@@ -0,0 +1,241 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/*
+ * Detects and reports unhandled rejections during test runs. Test harnesses
+ * will fail tests in this case, unless the test whitelists itself.
+ */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = [
+  "PromiseTestUtils",
+];
+
+const { classes: Cc, interfaces: Ci, utils: Cu, results: Cr } = Components;
+
+Cu.import("resource://gre/modules/Services.jsm", this);
+
+// Keep "JSMPromise" separate so "Promise" still refers to DOM Promises.
+let JSMPromise = Cu.import("resource://gre/modules/Promise.jsm", {}).Promise;
+
+// For now, we need test harnesses to provide a reference to Assert.jsm.
+let Assert = null;
+
+this.PromiseTestUtils = {
+  /**
+   * Array of objects containing the details of the Promise rejections that are
+   * currently left uncaught. This includes DOM Promise and Promise.jsm. When
+   * rejections in DOM Promises are consumed, they are removed from this list.
+   *
+   * The objects contain at least the following properties:
+   * {
+   *   message: The error message associated with the rejection, if any.
+   *   date: Date object indicating when the rejection was observed.
+   *   id: For DOM Promise only, the Promise ID from PromiseDebugging. This is
+   *       only used for tracking and should not be checked by the callers.
+   *   stack: nsIStackFrame, SavedFrame, or string indicating the stack at the
+   *          time the rejection was triggered. May also be null if the
+   *          rejection was triggered while a script was on the stack.
+   * }
+   */
+  _rejections: [],
+
+  /**
+   * When an uncaught rejection is detected, it is ignored if one of the
+   * functions in this array returns true when called with the rejection details
+   * as its only argument. When a function matches an expected rejection, it is
+   * then removed from the array.
+   */
+  _rejectionIgnoreFns: [],
+
+  /**
+   * Called only by the test infrastructure, registers the rejection observers.
+   *
+   * This should be called only once, and a matching "uninit" call must be made
+   * or the tests will crash on shutdown.
+   */
+  init() {
+    if (this._initialized) {
+      Cu.reportError("This object was already initialized.");
+      return;
+    }
+
+    PromiseDebugging.addUncaughtRejectionObserver(this);
+
+    // Promise.jsm rejections are only reported to this observer when requested,
+    // so we don't have to store a key to remove them when consumed.
+    JSMPromise.Debugging.addUncaughtErrorObserver(
+                            rejection => this._rejections.push(rejection));
+
+    this._initialized = true;
+  },
+  _initialized: false,
+
+  /**
+   * Called only by the test infrastructure, unregisters the observers.
+   */
+  uninit() {
+    if (!this._initialized) {
+      return;
+    }
+
+    PromiseDebugging.removeUncaughtRejectionObserver(this);
+    JSMPromise.Debugging.clearUncaughtErrorObservers();
+
+    this._initialized = false;
+  },
+
+  /**
+   * Called only by the test infrastructure, spins the event loop until the
+   * messages for pending DOM Promise rejections have been processed.
+   */
+  ensureDOMPromiseRejectionsProcessed() {
+    let observed = false;
+    let observer = {
+      onLeftUncaught: promise => {
+        if (PromiseDebugging.getState(promise).reason ===
+            this._ensureDOMPromiseRejectionsProcessedReason) {
+          observed = true;
+        }
+      },
+      onConsumed() {},
+    };
+
+    PromiseDebugging.addUncaughtRejectionObserver(observer);
+    Promise.reject(this._ensureDOMPromiseRejectionsProcessedReason);
+    while (!observed) {
+      Services.tm.mainThread.processNextEvent(true);
+    }
+    PromiseDebugging.removeUncaughtRejectionObserver(observer);
+  },
+  _ensureDOMPromiseRejectionsProcessedReason: {},
+
+  /**
+   * Called only by the tests for PromiseDebugging.addUncaughtRejectionObserver
+   * and for JSMPromise.Debugging, disables the observers in this module.
+   */
+  disableUncaughtRejectionObserverForSelfTest() {
+    this.uninit();
+  },
+
+  /**
+   * Called by tests that have been whitelisted, disables the observers in this
+   * module. For new tests where uncaught rejections are expected, you should
+   * use the more granular expectUncaughtRejection function instead.
+   */
+  thisTestLeaksUncaughtRejectionsAndShouldBeFixed() {
+    this.uninit();
+  },
+
+  /**
+   * Sets or updates the Assert object instance to be used for error reporting.
+   */
+  set Assert(assert) {
+    Assert = assert;
+  },
+
+  // UncaughtRejectionObserver
+  onLeftUncaught(promise) {
+    let message = "(Unable to convert rejection reason to string.)";
+    try {
+      let reason = PromiseDebugging.getState(promise).reason;
+      if (reason === this._ensureDOMPromiseRejectionsProcessedReason) {
+        // Ignore the special promise for ensureDOMPromiseRejectionsProcessed.
+        return;
+      }
+      message = reason.message || ("" + reason);
+    } catch (ex) {}
+
+    // It's important that we don't store any reference to the provided Promise
+    // object or its value after this function returns in order to avoid leaks.
+    this._rejections.push({
+      id: PromiseDebugging.getPromiseID(promise),
+      message,
+      date: new Date(),
+      stack: PromiseDebugging.getRejectionStack(promise),
+    });
+  },
+
+  // UncaughtRejectionObserver
+  onConsumed(promise) {
+    // We don't expect that many unhandled rejections will appear at the same
+    // time, so the algorithm doesn't need to be optimized for that case.
+    let id = PromiseDebugging.getPromiseID(promise);
+    let index = this._rejections.findIndex(rejection => rejection.id == id);
+    // If we get a consumption notification for a rejection that was left
+    // uncaught before this module was initialized, we can safely ignore it.
+    if (index != -1) {
+      this._rejections.splice(index, 1);
+    }
+  },
+
+  /**
+   * Informs the test suite that the test code will generate a Promise rejection
+   * that will still be unhandled when the test file terminates.
+   *
+   * This method must be called once for each instance of Promise that is
+   * expected to be uncaught, even if the rejection reason is the same for each
+   * instance.
+   *
+   * If the expected rejection does not occur, the test will fail.
+   *
+   * @param regExpOrCheckFn
+   *        This can either be a regular expression that should match the error
+   *        message of the rejection, or a check function that is invoked with
+   *        the rejection details object as its first argument.
+   */
+  expectUncaughtRejection(regExpOrCheckFn) {
+    let checkFn = !("test" in regExpOrCheckFn) ? regExpOrCheckFn :
+                  rejection => regExpOrCheckFn.test(rejection.message);
+    this._rejectionIgnoreFns.push(checkFn);
+  },
+
+  /**
+   * Fails the test if there are any uncaught rejections at this time that have
+   * not been whitelisted using expectUncaughtRejection.
+   *
+   * Depending on the configuration of the test suite, this function might only
+   * report the details of the first uncaught rejection that was generated.
+   *
+   * This is called by the test suite at the end of each test function.
+   */
+  assertNoUncaughtRejections() {
+    // Ask Promise.jsm to report all uncaught rejections to the observer now.
+    JSMPromise.Debugging.flushUncaughtErrors();
+
+    // If there is any uncaught rejection left at this point, the test fails.
+    while (this._rejections.length > 0) {
+      let rejection = this._rejections.shift();
+
+      // If one of the ignore functions matches, ignore the rejection, then
+      // remove the function so that each function only matches one rejection.
+      let index = this._rejectionIgnoreFns.findIndex(f => f(rejection));
+      if (index != -1) {
+        this._rejectionIgnoreFns.splice(index, 1);
+        continue;
+      }
+
+      // Report the error. This operation can throw an exception, depending on
+      // the configuration of the test suite that handles the assertion.
+      Assert.ok(false,
+                `A promise chain failed to handle a rejection:` +
+                ` ${rejection.message} - rejection date: ${rejection.date}`+
+                ` - stack: ${rejection.stack}`);
+    }
+  },
+
+  /**
+   * Fails the test if any rejection indicated by expectUncaughtRejection has
+   * not yet been reported at this time.
+   *
+   * This is called by the test suite at the end of each test file.
+   */
+  assertNoMoreExpectedRejections() {
+    // Only log this condition is there is a failure.
+    if (this._rejectionIgnoreFns.length > 0) {
+      Assert.equal(this._rejectionIgnoreFns.length, 0,
+             "Unable to find a rejection expected by expectUncaughtRejection.");
+    }
+  },
+};
diff --git a/toolkit/modules/tests/xpcshell/test_ObjectUtils_strict.js b/toolkit/modules/tests/xpcshell/test_ObjectUtils_strict.js
index b3a8863cda9..44572e6001a 100644
--- a/toolkit/modules/tests/xpcshell/test_ObjectUtils_strict.js
+++ b/toolkit/modules/tests/xpcshell/test_ObjectUtils_strict.js
@@ -1,12 +1,7 @@
 "use strict";
 
 var {ObjectUtils} = Components.utils.import("resource://gre/modules/ObjectUtils.jsm", {});
-var {Promise} = Components.utils.import("resource://gre/modules/Promise.jsm", {});
-
-add_task(function* init() {
-  // The code will cause uncaught rejections on purpose.
-  Promise.Debugging.clearUncaughtErrorObservers();
-});
+var {PromiseTestUtils} = Components.utils.import("resource://testing-common/PromiseTestUtils.jsm", {});
 
 add_task(function* test_strict() {
   let loose = { a: 1 };
@@ -16,11 +11,13 @@ add_task(function* test_strict() {
   loose.b || undefined; // Should not throw.
 
   strict.a; // Should not throw.
+  PromiseTestUtils.expectUncaughtRejection(/No such property: "b"/);
   Assert.throws(() => strict.b, /No such property: "b"/);
   "b" in strict; // Should not throw.
   strict.b = 2;
   strict.b; // Should not throw.
 
+  PromiseTestUtils.expectUncaughtRejection(/No such property: "c"/);
   Assert.throws(() => strict.c, /No such property: "c"/);
   "c" in strict; // Should not throw.
   loose.c = 3;
diff --git a/toolkit/modules/tests/xpcshell/test_Promise.js b/toolkit/modules/tests/xpcshell/test_Promise.js
index ab2f3b23384..6f39bb0fd2e 100644
--- a/toolkit/modules/tests/xpcshell/test_Promise.js
+++ b/toolkit/modules/tests/xpcshell/test_Promise.js
@@ -5,10 +5,10 @@
 Components.utils.import("resource://gre/modules/Promise.jsm");
 Components.utils.import("resource://gre/modules/Services.jsm");
 Components.utils.import("resource://gre/modules/Task.jsm");
+Components.utils.import("resource://testing-common/PromiseTestUtils.jsm");
 
-// Deactivate the standard xpcshell observer, as it turns uncaught
-// rejections into failures, which we don't want here.
-Promise.Debugging.clearUncaughtErrorObservers();
+// Prevent test failures due to the unhandled rejections in this test file.
+PromiseTestUtils.disableUncaughtRejectionObserverForSelfTest();
 
 ////////////////////////////////////////////////////////////////////////////////
 //// Test runner
diff --git a/toolkit/modules/tests/xpcshell/test_PromiseUtils.js b/toolkit/modules/tests/xpcshell/test_PromiseUtils.js
index ff9ba3688b5..5f1ac8b7d0b 100644
--- a/toolkit/modules/tests/xpcshell/test_PromiseUtils.js
+++ b/toolkit/modules/tests/xpcshell/test_PromiseUtils.js
@@ -5,6 +5,7 @@
 
 Components.utils.import("resource://gre/modules/PromiseUtils.jsm");
 Components.utils.import("resource://gre/modules/Timer.jsm");
+Components.utils.import("resource://testing-common/PromiseTestUtils.jsm");
 
 // Tests for PromiseUtils.jsm
 function run_test() {
@@ -98,8 +99,9 @@ add_task(function* test_reject_resolved_promise() {
 /* Test for the case when a rejected Promise is
  * passed to the reject method */
 add_task(function* test_reject_resolved_promise() {
+  PromiseTestUtils.expectUncaughtRejection(/This one rejects/);
   let def = PromiseUtils.defer();
-  let p = new Promise((resolve, reject) => reject(new Error("This on rejects")));
+  let p = new Promise((resolve, reject) => reject(new Error("This one rejects")));
   def.reject(p);
   yield Assert.rejects(def.promise, Promise, "Rejection with a rejected promise uses the passed promise itself as the reason of rejection");
 });
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_update_webextensions.js b/toolkit/mozapps/extensions/test/xpcshell/test_update_webextensions.js
index 5da4ecc6174..c0d4940b3bd 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_update_webextensions.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_update_webextensions.js
@@ -1,5 +1,17 @@
 "use strict";
 
+Components.utils.import("resource://testing-common/PromiseTestUtils.jsm", this);
+
+///////////////////
+//
+// Whitelisting this test.
+// As part of bug 1077403, the leaking uncaught rejection should be fixed.
+//
+// thisTestLeaksUncaughtRejectionsAndShouldBeFixed
+if (!TEST_UNPACKED) {
+  PromiseTestUtils.expectUncaughtRejection(/NS_ERROR_FILE_NOT_FOUND/);
+}
+
 const TOOLKIT_ID = "toolkit@mozilla.org";
 
 // We don't have an easy way to serve update manifests from a secure URL.
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_webextension.js b/toolkit/mozapps/extensions/test/xpcshell/test_webextension.js
index 28ed1177c60..c290486e2e1 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_webextension.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_webextension.js
@@ -2,6 +2,21 @@
  * http://creativecommons.org/publicdomain/zero/1.0/
  */
 
+Components.utils.import("resource://testing-common/PromiseTestUtils.jsm", this);
+
+///////////////////
+//
+// Whitelisting this test.
+// As part of bug 1077403, the leaking uncaught rejection should be fixed.
+//
+// thisTestLeaksUncaughtRejectionsAndShouldBeFixed
+if (TEST_UNPACKED) {
+  let codeAsString = ("" + Components.results.NS_ERROR_FILE_NOT_FOUND);
+  PromiseTestUtils.expectUncaughtRejection(r => r.message == codeAsString);
+} else {
+  PromiseTestUtils.expectUncaughtRejection(/Failed to open input source/);
+}
+
 const ID = "webextension1@tests.mozilla.org";
 
 const PREF_SELECTED_LOCALE = "general.useragent.locale";
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_webextension_icons.js b/toolkit/mozapps/extensions/test/xpcshell/test_webextension_icons.js
index 6f113b6c859..37f51b09f77 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_webextension_icons.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_webextension_icons.js
@@ -2,6 +2,23 @@
  * http://creativecommons.org/publicdomain/zero/1.0/
  */
 
+Components.utils.import("resource://testing-common/PromiseTestUtils.jsm", this);
+
+///////////////////
+//
+// Whitelisting this test.
+// As part of bug 1077403, the leaking uncaught rejection should be fixed.
+//
+// thisTestLeaksUncaughtRejectionsAndShouldBeFixed
+for (let i = 0; i < 4; i++) {
+  if (TEST_UNPACKED) {
+    let codeAsString = ("" + Components.results.NS_ERROR_FILE_NOT_FOUND);
+    PromiseTestUtils.expectUncaughtRejection(r => r.message == codeAsString);
+  } else {
+    PromiseTestUtils.expectUncaughtRejection(/Failed to open input source/);
+  }
+}
+
 const ID = "webextension1@tests.mozilla.org";
 
 const profileDir = gProfD.clone();