diff --git a/dom/base/Console.cpp b/dom/base/Console.cpp
index b09396a34d4d..e563cdc0d7a0 100644
--- a/dom/base/Console.cpp
+++ b/dom/base/Console.cpp
@@ -371,7 +371,7 @@ private:
 
     RefPtr<WorkerControlRunnable> runnable =
       new ConsoleReleaseRunnable(mWorkerPrivate, this);
-    runnable->Dispatch(nullptr);
+    runnable->Dispatch();
   }
 
   void
diff --git a/dom/base/WebSocket.cpp b/dom/base/WebSocket.cpp
index 4cc13b24f5ff..a605902dd41c 100644
--- a/dom/base/WebSocket.cpp
+++ b/dom/base/WebSocket.cpp
@@ -2546,7 +2546,7 @@ WebSocketImpl::Cancel(nsresult aStatus)
     MOZ_ASSERT(mWorkerPrivate);
     RefPtr<CancelRunnable> runnable =
       new CancelRunnable(mWorkerPrivate, this);
-    if (!runnable->Dispatch(nullptr)) {
+    if (!runnable->Dispatch()) {
       return NS_ERROR_FAILURE;
     }
 
@@ -2749,7 +2749,7 @@ WebSocketImpl::Dispatch(already_AddRefed<nsIRunnable>&& aEvent, uint32_t aFlags)
   RefPtr<WorkerRunnableDispatcher> event =
     new WorkerRunnableDispatcher(this, mWorkerPrivate, event_ref.forget());
 
-  if (!event->Dispatch(nullptr)) {
+  if (!event->Dispatch()) {
     return NS_ERROR_FAILURE;
   }
 
diff --git a/dom/canvas/ImageBitmap.cpp b/dom/canvas/ImageBitmap.cpp
index 6b6d7826b8f0..98856168ebce 100644
--- a/dom/canvas/ImageBitmap.cpp
+++ b/dom/canvas/ImageBitmap.cpp
@@ -894,7 +894,7 @@ AsyncFulfillImageBitmapPromise(Promise* aPromise, ImageBitmap* aImageBitmap)
   } else {
     RefPtr<FulfillImageBitmapPromiseWorkerTask> task =
       new FulfillImageBitmapPromiseWorkerTask(aPromise, aImageBitmap);
-    task->Dispatch(GetCurrentThreadWorkerPrivate()->GetJSContext()); // Actually, to the current worker-thread.
+    task->Dispatch(); // Actually, to the current worker-thread.
   }
 }
 
@@ -1167,7 +1167,7 @@ AsyncCreateImageBitmapFromBlob(Promise* aPromise, nsIGlobalObject* aGlobal,
   } else {
     RefPtr<CreateImageBitmapFromBlobWorkerTask> task =
       new CreateImageBitmapFromBlobWorkerTask(aPromise, aGlobal, aBlob, aCropRect);
-    task->Dispatch(GetCurrentThreadWorkerPrivate()->GetJSContext()); // Actually, to the current worker-thread.
+    task->Dispatch(); // Actually, to the current worker-thread.
   }
 }
 
diff --git a/dom/fetch/Fetch.cpp b/dom/fetch/Fetch.cpp
index 3a4732820710..779fb3aede06 100644
--- a/dom/fetch/Fetch.cpp
+++ b/dom/fetch/Fetch.cpp
@@ -326,9 +326,7 @@ WorkerFetchResolver::OnResponseAvailableInternal(InternalResponse* aResponse)
     new WorkerFetchResponseRunnable(mPromiseProxy->GetWorkerPrivate(), this,
                                     aResponse);
 
-  AutoJSAPI jsapi;
-  jsapi.Init();
-  if (!r->Dispatch(jsapi.cx())) {
+  if (!r->Dispatch()) {
     NS_WARNING("Could not dispatch fetch response");
   }
 }
@@ -345,9 +343,7 @@ WorkerFetchResolver::OnResponseEnd()
   RefPtr<WorkerFetchResponseEndRunnable> r =
     new WorkerFetchResponseEndRunnable(mPromiseProxy->GetWorkerPrivate(), this);
 
-  AutoJSAPI jsapi;
-  jsapi.Init();
-  if (!r->Dispatch(jsapi.cx())) {
+  if (!r->Dispatch()) {
     NS_WARNING("Could not dispatch fetch response end");
   }
 }
@@ -611,8 +607,7 @@ public:
       if (mBody->mWorkerPrivate) {
         RefPtr<FailConsumeBodyWorkerRunnable<Derived>> r =
           new FailConsumeBodyWorkerRunnable<Derived>(mBody);
-        AutoSafeJSContext cx;
-        if (!r->Dispatch(cx)) {
+        if (!r->Dispatch()) {
           MOZ_CRASH("We are going to leak");
         }
       } else {
@@ -662,8 +657,7 @@ public:
                                         aStatus,
                                         aResultLength,
                                         nonconstResult);
-      AutoSafeJSContext cx;
-      if (!r->Dispatch(cx)) {
+      if (!r->Dispatch()) {
         // XXXcatalinb: The worker is shutting down, the pump will be canceled
         // by FetchBodyFeature::Notify.
         NS_WARNING("Could not dispatch ConsumeBodyRunnable");
diff --git a/dom/indexedDB/ActorsChild.cpp b/dom/indexedDB/ActorsChild.cpp
index 17d2e01811e1..3e9487ea11aa 100644
--- a/dom/indexedDB/ActorsChild.cpp
+++ b/dom/indexedDB/ActorsChild.cpp
@@ -943,7 +943,7 @@ public:
       RefPtr<WorkerPermissionOperationCompleted> runnable =
         new WorkerPermissionOperationCompleted(mWorkerPrivate, this);
 
-      MOZ_ALWAYS_TRUE(runnable->Dispatch(nullptr));
+      MOZ_ALWAYS_TRUE(runnable->Dispatch());
       return;
     }
 
diff --git a/dom/notification/Notification.cpp b/dom/notification/Notification.cpp
index b1f717d5a46f..bd8b0ff879b5 100644
--- a/dom/notification/Notification.cpp
+++ b/dom/notification/Notification.cpp
@@ -503,12 +503,10 @@ public:
         RefPtr<ReleaseNotificationRunnable> r =
           new ReleaseNotificationRunnable(notification);
 
-        AutoJSAPI jsapi;
-        jsapi.Init();
-        if (!r->Dispatch(jsapi.cx())) {
+        if (!r->Dispatch()) {
           RefPtr<ReleaseNotificationControlRunnable> r =
             new ReleaseNotificationControlRunnable(notification);
-          MOZ_ALWAYS_TRUE(r->Dispatch(jsapi.cx()));
+          MOZ_ALWAYS_TRUE(r->Dispatch());
         }
       } else {
         notification->AssertIsOnTargetThread();
@@ -1554,8 +1552,7 @@ WorkerNotificationObserver::Observe(nsISupports* aSubject, const char* aTopic,
   }
 
   MOZ_ASSERT(r);
-  AutoSafeJSContext cx;
-  if (!r->Dispatch(cx)) {
+  if (!r->Dispatch()) {
     NS_WARNING("Could not dispatch event to worker notification");
   }
   return NS_OK;
@@ -1726,8 +1723,7 @@ Notification::ShowInternal()
       RefPtr<NotificationEventWorkerRunnable> r =
         new NotificationEventWorkerRunnable(this,
                                             NS_LITERAL_STRING("error"));
-      AutoSafeJSContext cx;
-      if (!r->Dispatch(cx)) {
+      if (!r->Dispatch()) {
         NS_WARNING("Could not dispatch event to worker notification");
       }
     } else {
@@ -2153,7 +2149,7 @@ public:
                                   proxy,
                                   Move(mStrings));
 
-    r->Dispatch(aCx);
+    r->Dispatch();
     return NS_OK;
   }
 
diff --git a/dom/promise/Promise.cpp b/dom/promise/Promise.cpp
index aee724c61aa5..c5513529ce4f 100644
--- a/dom/promise/Promise.cpp
+++ b/dom/promise/Promise.cpp
@@ -2905,7 +2905,7 @@ PromiseWorkerProxy::RunCallback(JSContext* aCx,
   RefPtr<PromiseWorkerProxyRunnable> runnable =
     new PromiseWorkerProxyRunnable(this, aFunc);
 
-  runnable->Dispatch(aCx);
+  runnable->Dispatch();
 }
 
 void
diff --git a/dom/promise/PromiseWorkerProxy.h b/dom/promise/PromiseWorkerProxy.h
index 32e8a3de4e5b..c3b3e451626b 100644
--- a/dom/promise/PromiseWorkerProxy.h
+++ b/dom/promise/PromiseWorkerProxy.h
@@ -84,9 +84,7 @@ class WorkerPrivate;
 //
 //        RefPtr<FinishTaskWorkerRunnable> runnable =
 //          new FinishTaskWorkerRunnable(proxy->GetWorkerPrivate(), proxy, result);
-//        AutoJSAPI jsapi;
-//        jsapi.Init();
-//        if (!r->Dispatch(jsapi.cx())) {
+//        if (!r->Dispatch()) {
 //          // Worker is alive but not Running any more, so the Promise can't
 //          // be resolved, give up. The proxy will get Release()d at some
 //          // point.
diff --git a/dom/push/PushManager.cpp b/dom/push/PushManager.cpp
index 54f8c9aba228..26edf18bcb1a 100644
--- a/dom/push/PushManager.cpp
+++ b/dom/push/PushManager.cpp
@@ -442,12 +442,9 @@ public:
       return NS_OK;
     }
 
-    AutoJSAPI jsapi;
-    jsapi.Init();
-
     RefPtr<UnsubscribeResultRunnable> r =
       new UnsubscribeResultRunnable(proxy, aStatus, aSuccess);
-    r->Dispatch(jsapi.cx());
+    r->Dispatch();
     return NS_OK;
   }
 
@@ -644,9 +641,6 @@ public:
       return NS_OK;
     }
 
-    AutoJSAPI jsapi;
-    jsapi.Init();
-
     nsAutoString endpoint;
     nsTArray<uint8_t> rawP256dhKey, authSecret;
     if (NS_SUCCEEDED(aStatus)) {
@@ -661,7 +655,7 @@ public:
                                         mScope,
                                         rawP256dhKey,
                                         authSecret);
-    r->Dispatch(jsapi.cx());
+    r->Dispatch();
     return NS_OK;
   }
 
@@ -913,11 +907,9 @@ public:
       state
     );
 
-    AutoJSAPI jsapi;
-    jsapi.Init();
     RefPtr<PermissionResultRunnable> r =
       new PermissionResultRunnable(mProxy, rv, state);
-    r->Dispatch(jsapi.cx());
+    r->Dispatch();
     return NS_OK;
   }
 
diff --git a/dom/workers/DataStore.cpp b/dom/workers/DataStore.cpp
index 6284b6388b22..ce5eec2a97f1 100644
--- a/dom/workers/DataStore.cpp
+++ b/dom/workers/DataStore.cpp
@@ -954,11 +954,7 @@ DataStoreChangeEventProxy::HandleEvent(nsIDOMEvent* aEvent)
   RefPtr<DispatchDataStoreChangeEventRunnable> runnable =
     new DispatchDataStoreChangeEventRunnable(this, event);
 
-  {
-    AutoSafeJSContext cx;
-    JSAutoRequest ar(cx);
-    runnable->Dispatch(cx);
-  }
+  runnable->Dispatch();
 
   return NS_OK;
 }
diff --git a/dom/workers/RuntimeService.cpp b/dom/workers/RuntimeService.cpp
index d6fc505a8514..71974ba408c3 100644
--- a/dom/workers/RuntimeService.cpp
+++ b/dom/workers/RuntimeService.cpp
@@ -601,7 +601,7 @@ public:
   NS_DECL_ISUPPORTS_INHERITED
 
   bool
-  Dispatch(JSContext* aCx)
+  Dispatch()
   {
     AutoSyncLoopHolder syncLoop(mWorkerPrivate);
 
@@ -609,7 +609,6 @@ public:
     MOZ_ASSERT(mSyncLoopTarget);
 
     if (NS_FAILED(NS_DispatchToMainThread(this))) {
-      JS_ReportError(aCx, "Failed to dispatch to main thread!");
       return false;
     }
 
@@ -642,9 +641,7 @@ ContentSecurityPolicyAllows(JSContext* aCx)
     RefPtr<LogViolationDetailsRunnable> runnable =
         new LogViolationDetailsRunnable(worker, fileName, lineNum);
 
-    if (!runnable->Dispatch(aCx)) {
-      JS_ReportPendingException(aCx);
-    }
+    runnable->Dispatch();
   }
 
   return worker->IsEvalAllowed();
@@ -1255,7 +1252,7 @@ WorkerCrossThreadDispatcher::PostTask(WorkerTask* aTask)
 
   RefPtr<WorkerTaskRunnable> runnable =
     new WorkerTaskRunnable(mWorkerPrivate, aTask);
-  return runnable->Dispatch(nullptr);
+  return runnable->Dispatch();
 }
 
 WorkerPrivate*
@@ -2587,7 +2584,7 @@ LogViolationDetailsRunnable::Run()
   RefPtr<MainThreadStopSyncLoopRunnable> response =
     new MainThreadStopSyncLoopRunnable(mWorkerPrivate, mSyncLoopTarget.forget(),
                                        true);
-  MOZ_ALWAYS_TRUE(response->Dispatch(nullptr));
+  MOZ_ALWAYS_TRUE(response->Dispatch());
 
   return NS_OK;
 }
diff --git a/dom/workers/ScriptLoader.cpp b/dom/workers/ScriptLoader.cpp
index 321b602622ea..663a968a4ba6 100644
--- a/dom/workers/ScriptLoader.cpp
+++ b/dom/workers/ScriptLoader.cpp
@@ -1252,7 +1252,7 @@ private:
       RefPtr<ScriptExecutorRunnable> runnable =
         new ScriptExecutorRunnable(*this, mSyncLoopTarget, IsMainWorkerScript(),
                                    firstIndex, lastIndex);
-      if (!runnable->Dispatch(nullptr)) {
+      if (!runnable->Dispatch()) {
         MOZ_ASSERT(false, "This should never fail!");
       }
     }
@@ -1682,7 +1682,7 @@ public:
     RefPtr<MainThreadStopSyncLoopRunnable> runnable =
       new MainThreadStopSyncLoopRunnable(mParentWorker,
                                          mSyncLoopTarget.forget(), true);
-    if (!runnable->Dispatch(nullptr)) {
+    if (!runnable->Dispatch()) {
       NS_ERROR("This should never fail!");
     }
 
diff --git a/dom/workers/ServiceWorkerClients.cpp b/dom/workers/ServiceWorkerClients.cpp
index 97477368fc83..0b3345a07b6e 100644
--- a/dom/workers/ServiceWorkerClients.cpp
+++ b/dom/workers/ServiceWorkerClients.cpp
@@ -132,9 +132,7 @@ public:
                                        rv.StealNSResult());
     rv.SuppressException();
 
-    AutoJSAPI jsapi;
-    jsapi.Init();
-    r->Dispatch(jsapi.cx());
+    r->Dispatch();
     return NS_OK;
   }
 };
@@ -212,9 +210,7 @@ public:
       new ResolvePromiseWorkerRunnable(mPromiseProxy->GetWorkerPrivate(),
                                        mPromiseProxy, result);
 
-    AutoJSAPI jsapi;
-    jsapi.Init();
-    r->Dispatch(jsapi.cx());
+    r->Dispatch();
     return NS_OK;
   }
 };
@@ -292,9 +288,7 @@ public:
     RefPtr<ResolveClaimRunnable> r =
       new ResolveClaimRunnable(workerPrivate, mPromiseProxy, rv);
 
-    AutoJSAPI jsapi;
-    jsapi.Init();
-    r->Dispatch(jsapi.cx());
+    r->Dispatch();
     return NS_OK;
   }
 };
@@ -405,10 +399,7 @@ public:
       new ResolveOpenWindowRunnable(mPromiseProxy,
                                     Move(clientInfo),
                                     NS_OK);
-    AutoJSAPI jsapi;
-    jsapi.Init();
-    JSContext* cx = jsapi.cx();
-    r->Dispatch(cx);
+    r->Dispatch();
 
     return NS_OK;
   }
@@ -553,9 +544,7 @@ public:
     RefPtr<ResolveOpenWindowRunnable> resolveRunnable =
       new ResolveOpenWindowRunnable(mPromiseProxy, nullptr, rv);
 
-    AutoJSAPI jsapi;
-    jsapi.Init();
-    NS_WARN_IF(!resolveRunnable->Dispatch(jsapi.cx()));
+    NS_WARN_IF(!resolveRunnable->Dispatch());
 
     return NS_OK;
   }
diff --git a/dom/workers/ServiceWorkerPrivate.cpp b/dom/workers/ServiceWorkerPrivate.cpp
index 510772344412..52b2c39ff402 100644
--- a/dom/workers/ServiceWorkerPrivate.cpp
+++ b/dom/workers/ServiceWorkerPrivate.cpp
@@ -163,9 +163,7 @@ ServiceWorkerPrivate::CheckScriptEvaluation(LifeCycleEventCallback* aCallback)
   RefPtr<WorkerRunnable> r = new CheckScriptEvaluationWithCallback(mWorkerPrivate,
                                                                    mKeepAliveToken,
                                                                    aCallback);
-  AutoJSAPI jsapi;
-  jsapi.Init();
-  if (NS_WARN_IF(!r->Dispatch(jsapi.cx()))) {
+  if (NS_WARN_IF(!r->Dispatch())) {
     return NS_ERROR_FAILURE;
   }
 
@@ -548,9 +546,7 @@ ServiceWorkerPrivate::SendLifeCycleEvent(const nsAString& aEventType,
                                                                 mKeepAliveToken,
                                                                 aEventType,
                                                                 aCallback);
-  AutoJSAPI jsapi;
-  jsapi.Init();
-  if (NS_WARN_IF(!r->Dispatch(jsapi.cx()))) {
+  if (NS_WARN_IF(!r->Dispatch())) {
     return NS_ERROR_FAILURE;
   }
 
@@ -680,9 +676,7 @@ ServiceWorkerPrivate::SendPushEvent(const Maybe<nsTArray<uint8_t>>& aData,
 
   MOZ_ASSERT(mInfo->State() == ServiceWorkerState::Activated);
 
-  AutoJSAPI jsapi;
-  jsapi.Init();
-  if (NS_WARN_IF(!r->Dispatch(jsapi.cx()))) {
+  if (NS_WARN_IF(!r->Dispatch())) {
     return NS_ERROR_FAILURE;
   }
 
@@ -702,9 +696,7 @@ ServiceWorkerPrivate::SendPushSubscriptionChangeEvent()
   MOZ_ASSERT(mKeepAliveToken);
   RefPtr<WorkerRunnable> r =
     new SendPushSubscriptionChangeEventRunnable(mWorkerPrivate, mKeepAliveToken);
-  AutoJSAPI jsapi;
-  jsapi.Init();
-  if (NS_WARN_IF(!r->Dispatch(jsapi.cx()))) {
+  if (NS_WARN_IF(!r->Dispatch())) {
     return NS_ERROR_FAILURE;
   }
 
@@ -976,9 +968,7 @@ ServiceWorkerPrivate::SendNotificationClickEvent(const nsAString& aID,
                                            aID, aTitle, aDir, aLang,
                                            aBody, aTag, aIcon, aData,
                                            aBehavior, aScope);
-  AutoJSAPI jsapi;
-  jsapi.Init();
-  if (NS_WARN_IF(!r->Dispatch(jsapi.cx()))) {
+  if (NS_WARN_IF(!r->Dispatch())) {
     return NS_ERROR_FAILURE;
   }
 
@@ -1351,9 +1341,7 @@ ServiceWorkerPrivate::SendFetchEvent(nsIInterceptedChannel* aChannel,
 
   MOZ_ASSERT(mInfo->State() == ServiceWorkerState::Activated);
 
-  AutoJSAPI jsapi;
-  jsapi.Init();
-  if (NS_WARN_IF(!r->Dispatch(jsapi.cx()))) {
+  if (NS_WARN_IF(!r->Dispatch())) {
     return NS_ERROR_FAILURE;
   }
 
@@ -1543,9 +1531,7 @@ ServiceWorkerPrivate::Activated()
 
   for (uint32_t i = 0; i < pendingEvents.Length(); ++i) {
     RefPtr<WorkerRunnable> r = pendingEvents[i].forget();
-    AutoJSAPI jsapi;
-    jsapi.Init();
-    if (NS_WARN_IF(!r->Dispatch(jsapi.cx()))) {
+    if (NS_WARN_IF(!r->Dispatch())) {
       NS_WARNING("Failed to dispatch pending functional event!");
     }
   }
diff --git a/dom/workers/ServiceWorkerRegistration.cpp b/dom/workers/ServiceWorkerRegistration.cpp
index 49419263da57..d5afaa34e8e0 100644
--- a/dom/workers/ServiceWorkerRegistration.cpp
+++ b/dom/workers/ServiceWorkerRegistration.cpp
@@ -388,12 +388,9 @@ public:
       return;
     }
 
-    AutoJSAPI jsapi;
-    jsapi.Init();
-
     RefPtr<UpdateResultRunnable> r =
       new UpdateResultRunnable(proxy, aStatus);
-    r->Dispatch(jsapi.cx());
+    r->Dispatch();
   }
 };
 
@@ -555,9 +552,7 @@ private:
     RefPtr<WorkerRunnable> r =
       new FulfillUnregisterPromiseRunnable(proxy, aState);
 
-    AutoJSAPI jsapi;
-    jsapi.Init();
-    r->Dispatch(jsapi.cx());
+    r->Dispatch();
   }
 };
 
@@ -1201,10 +1196,7 @@ WorkerListener::UpdateFound()
   if (mWorkerPrivate) {
     RefPtr<FireUpdateFoundRunnable> r =
       new FireUpdateFoundRunnable(mWorkerPrivate, this);
-    AutoJSAPI jsapi;
-    jsapi.Init();
-    if (NS_WARN_IF(!r->Dispatch(jsapi.cx()))) {
-    }
+    NS_WARN_IF(!r->Dispatch());
   }
 }
 
diff --git a/dom/workers/ServiceWorkerWindowClient.cpp b/dom/workers/ServiceWorkerWindowClient.cpp
index a1b2c3e4629b..a5e3e0f9e2f4 100644
--- a/dom/workers/ServiceWorkerWindowClient.cpp
+++ b/dom/workers/ServiceWorkerWindowClient.cpp
@@ -118,9 +118,7 @@ private:
       new ResolveOrRejectPromiseRunnable(mPromiseProxy->GetWorkerPrivate(),
                                          mPromiseProxy, Move(aClientInfo));
 
-    AutoJSAPI jsapi;
-    jsapi.Init();
-    resolveRunnable->Dispatch(jsapi.cx());
+    resolveRunnable->Dispatch();
   }
 };
 
diff --git a/dom/workers/WorkerPrivate.cpp b/dom/workers/WorkerPrivate.cpp
index cbd115477c9d..3a950ab70867 100644
--- a/dom/workers/WorkerPrivate.cpp
+++ b/dom/workers/WorkerPrivate.cpp
@@ -466,14 +466,6 @@ public:
     mIncrease(aIncrease)
   { }
 
-  // We don't need a JSContext to dispatch, since we know our PreDispatch and
-  // DispatchInternal don't throw any exceptions.
-  bool
-  Dispatch()
-  {
-    return WorkerControlRunnable::Dispatch(nullptr);
-  }
-
 private:
   virtual bool
   WorkerRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate) override
@@ -853,14 +845,6 @@ public:
                aStatus == Canceling || aStatus == Killing);
   }
 
-  // We can be dispatched without a JSContext, because all we do with the
-  // JSContext passed to Dispatch() normally for worker runnables is call
-  // ModifyBusyCount... but that doesn't actually use its JSContext argument.
-  bool Dispatch()
-  {
-    return WorkerControlRunnable::Dispatch(nullptr);
-  }
-
 private:
   virtual bool
   PreDispatch(WorkerPrivate* aWorkerPrivate) override
@@ -1070,7 +1054,7 @@ public:
         new ReportErrorRunnable(aWorkerPrivate, aMessage, aFilename, aLine,
                                 aLineNumber, aColumnNumber, aFlags,
                                 aErrorNumber, aExnType, aMutedError);
-      return runnable->Dispatch(aCx);
+      return runnable->Dispatch();
     }
 
     // Otherwise log an error to the error console.
@@ -1755,7 +1739,7 @@ TimerThreadEventTarget::Dispatch(already_AddRefed<nsIRunnable>&& aRunnable, uint
 
   // This can fail if we're racing to terminate or cancel, should be handled
   // by the terminate or cancel code.
-  mWorkerRunnable->Dispatch(nullptr);
+  mWorkerRunnable->Dispatch();
 
   return NS_OK;
 }
@@ -2633,7 +2617,7 @@ WorkerPrivateParent<Derived>::Freeze(JSContext* aCx,
 
   RefPtr<FreezeRunnable> runnable =
     new FreezeRunnable(ParentAsWorkerPrivate());
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     return false;
   }
 
@@ -2715,7 +2699,7 @@ WorkerPrivateParent<Derived>::Thaw(JSContext* aCx, nsPIDOMWindowInner* aWindow)
 
   RefPtr<ThawRunnable> runnable =
     new ThawRunnable(ParentAsWorkerPrivate());
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     return false;
   }
 
@@ -2927,7 +2911,7 @@ WorkerPrivateParent<Derived>::PostMessageInternal(
 
   runnable->SetMessageSource(Move(aClientInfo));
 
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     aRv.Throw(NS_ERROR_FAILURE);
   }
 }
@@ -2969,9 +2953,8 @@ WorkerPrivateParent<Derived>::UpdateRuntimeOptions(
 
   RefPtr<UpdateRuntimeOptionsRunnable> runnable =
     new UpdateRuntimeOptionsRunnable(ParentAsWorkerPrivate(), aRuntimeOptions);
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     NS_WARNING("Failed to update worker context options!");
-    JS_ClearPendingException(aCx);
   }
 }
 
@@ -2984,9 +2967,8 @@ WorkerPrivateParent<Derived>::UpdatePreference(JSContext* aCx, WorkerPreference
 
   RefPtr<UpdatePreferenceRunnable> runnable =
     new UpdatePreferenceRunnable(ParentAsWorkerPrivate(), aPref, aValue);
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     NS_WARNING("Failed to update worker preferences!");
-    JS_ClearPendingException(aCx);
   }
 }
 
@@ -2999,9 +2981,8 @@ WorkerPrivateParent<Derived>::UpdateLanguages(JSContext* aCx,
 
   RefPtr<UpdateLanguagesRunnable> runnable =
     new UpdateLanguagesRunnable(ParentAsWorkerPrivate(), aLanguages);
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     NS_WARNING("Failed to update worker languages!");
-    JS_ClearPendingException(aCx);
   }
 }
 
@@ -3024,9 +3005,8 @@ WorkerPrivateParent<Derived>::UpdateJSWorkerMemoryParameter(JSContext* aCx,
     RefPtr<UpdateJSWorkerMemoryParameterRunnable> runnable =
       new UpdateJSWorkerMemoryParameterRunnable(ParentAsWorkerPrivate(), aKey,
                                                 aValue);
-    if (!runnable->Dispatch(aCx)) {
+    if (!runnable->Dispatch()) {
       NS_WARNING("Failed to update memory parameter!");
-      JS_ClearPendingException(aCx);
     }
   }
 }
@@ -3047,9 +3027,8 @@ WorkerPrivateParent<Derived>::UpdateGCZeal(JSContext* aCx, uint8_t aGCZeal,
 
   RefPtr<UpdateGCZealRunnable> runnable =
     new UpdateGCZealRunnable(ParentAsWorkerPrivate(), aGCZeal, aFrequency);
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     NS_WARNING("Failed to update worker gczeal!");
-    JS_ClearPendingException(aCx);
   }
 }
 #endif
@@ -3063,9 +3042,8 @@ WorkerPrivateParent<Derived>::GarbageCollect(JSContext* aCx, bool aShrinking)
   RefPtr<GarbageCollectRunnable> runnable =
     new GarbageCollectRunnable(ParentAsWorkerPrivate(), aShrinking,
                                /* collectChildren = */ true);
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     NS_WARNING("Failed to GC worker!");
-    JS_ClearPendingException(aCx);
   }
 }
 
@@ -3078,9 +3056,8 @@ WorkerPrivateParent<Derived>::CycleCollect(JSContext* aCx, bool aDummy)
   RefPtr<CycleCollectRunnable> runnable =
     new CycleCollectRunnable(ParentAsWorkerPrivate(),
                              /* collectChildren = */ true);
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     NS_WARNING("Failed to CC worker!");
-    JS_ClearPendingException(aCx);
   }
 }
 
@@ -3092,9 +3069,8 @@ WorkerPrivateParent<Derived>::OfflineStatusChangeEvent(JSContext* aCx, bool aIsO
 
   RefPtr<OfflineStatusChangeRunnable> runnable =
     new OfflineStatusChangeRunnable(ParentAsWorkerPrivate(), aIsOffline);
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     NS_WARNING("Failed to dispatch offline status change event!");
-    JS_ClearPendingException(aCx);
   }
 }
 
@@ -3148,7 +3124,7 @@ WorkerPrivateParent<Derived>::RegisterSharedWorker(JSContext* aCx,
   if (IsSharedWorker()) {
     RefPtr<MessagePortRunnable> runnable =
       new MessagePortRunnable(ParentAsWorkerPrivate(), aPort);
-    if (!runnable->Dispatch(aCx)) {
+    if (!runnable->Dispatch()) {
       return false;
     }
   }
@@ -3810,7 +3786,7 @@ WorkerDebugger::Initialize(const nsAString& aURL, JSContext* aCx)
   if (!mIsInitialized) {
     RefPtr<CompileDebuggerScriptRunnable> runnable =
       new CompileDebuggerScriptRunnable(mWorkerPrivate, aURL);
-    if (!runnable->Dispatch(aCx)) {
+    if (!runnable->Dispatch()) {
       return NS_ERROR_FAILURE;
     }
 
@@ -3831,7 +3807,7 @@ WorkerDebugger::PostMessageMoz(const nsAString& aMessage, JSContext* aCx)
 
   RefPtr<DebuggerMessageEventRunnable> runnable =
     new DebuggerMessageEventRunnable(mWorkerPrivate, aMessage);
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     return NS_ERROR_FAILURE;
   }
 
@@ -4118,7 +4094,7 @@ WorkerPrivate::Constructor(JSContext* aCx,
 
   RefPtr<CompileScriptRunnable> compiler =
     new CompileScriptRunnable(worker, aScriptURL);
-  if (!compiler->Dispatch(aCx)) {
+  if (!compiler->Dispatch()) {
     aRv.Throw(NS_ERROR_UNEXPECTED);
     return nullptr;
   }
@@ -4778,7 +4754,7 @@ WorkerPrivate::ScheduleDeletion(WorkerRanOrNot aRanOrNot)
   if (WorkerPrivate* parent = GetParent()) {
     RefPtr<WorkerFinishedRunnable> runnable =
       new WorkerFinishedRunnable(parent, this);
-    if (!runnable->Dispatch(nullptr)) {
+    if (!runnable->Dispatch()) {
       NS_WARNING("Failed to dispatch runnable!");
     }
   }
@@ -5504,7 +5480,7 @@ WorkerPrivate::PostMessageToParentInternal(
     return;
   }
 
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     aRv = NS_ERROR_FAILURE;
   }
 }
@@ -5595,7 +5571,7 @@ WorkerPrivate::SetDebuggerImmediate(JSContext* aCx, dom::Function& aHandler,
 
   RefPtr<DebuggerImmediateRunnable> runnable =
     new DebuggerImmediateRunnable(this, aHandler);
-  if (!runnable->Dispatch(aCx)) {
+  if (!runnable->Dispatch()) {
     aRv.Throw(NS_ERROR_FAILURE);
   }
 }
@@ -5697,7 +5673,7 @@ WorkerPrivate::NotifyInternal(JSContext* aCx, Status aStatus)
   if (aStatus == Closing) {
     // Notify parent to stop sending us messages and balance our busy count.
     RefPtr<CloseRunnable> runnable = new CloseRunnable(this);
-    if (!runnable->Dispatch(aCx)) {
+    if (!runnable->Dispatch()) {
       return false;
     }
 
diff --git a/dom/workers/WorkerRunnable.cpp b/dom/workers/WorkerRunnable.cpp
index baaaf8b69e27..411f34e1b7b8 100644
--- a/dom/workers/WorkerRunnable.cpp
+++ b/dom/workers/WorkerRunnable.cpp
@@ -87,41 +87,13 @@ WorkerRunnable::PreDispatch(WorkerPrivate* aWorkerPrivate)
 }
 
 bool
-WorkerRunnable::Dispatch(JSContext* aCx)
+WorkerRunnable::Dispatch()
 {
-  bool ok;
-
-  if (!aCx) {
-    ok = PreDispatch(mWorkerPrivate);
-    if (ok) {
-      ok = DispatchInternal();
-    }
-    PostDispatch(mWorkerPrivate, ok);
-    return ok;
+  bool ok = PreDispatch(mWorkerPrivate);
+  if (ok) {
+    ok = DispatchInternal();
   }
-
-  JSAutoRequest ar(aCx);
-
-  JS::Rooted<JSObject*> global(aCx, JS::CurrentGlobalOrNull(aCx));
-
-  Maybe<JSAutoCompartment> ac;
-  if (global) {
-    ac.emplace(aCx, global);
-  }
-
-  MOZ_ASSERT(!JS_IsExceptionPending(aCx));
-
-  ok = PreDispatch(mWorkerPrivate);
-  MOZ_ASSERT(!JS_IsExceptionPending(aCx));
-
-  if (ok && !DispatchInternal()) {
-    ok = false;
-  }
-  MOZ_ASSERT(!JS_IsExceptionPending(aCx));
-
   PostDispatch(mWorkerPrivate, ok);
-  MOZ_ASSERT(!JS_IsExceptionPending(aCx));
-
   return ok;
 }
 
@@ -599,7 +571,7 @@ WorkerMainThreadRunnable::Run()
                                        mSyncLoopTarget.forget(),
                                        runResult);
 
-  MOZ_ALWAYS_TRUE(response->Dispatch(nullptr));
+  MOZ_ALWAYS_TRUE(response->Dispatch());
 
   return NS_OK;
 }
diff --git a/dom/workers/WorkerRunnable.h b/dom/workers/WorkerRunnable.h
index 189e372710ff..860df2b9f59f 100644
--- a/dom/workers/WorkerRunnable.h
+++ b/dom/workers/WorkerRunnable.h
@@ -73,12 +73,10 @@ public:
   // appropriately.
   NS_DECL_NSICANCELABLERUNNABLE
 
-  // Passing a JSContext here is required for the WorkerThreadModifyBusyCount
-  // behavior. It also guarantees that any failure (false return) will throw an
-  // exception on the given context. If a context is not passed then failures
-  // must be dealt with by the caller.
+  // The return value is true if and only if both PreDispatch and
+  // DispatchInternal return true.
   bool
-  Dispatch(JSContext* aCx);
+  Dispatch();
 
   // See above note about Cancel().
   virtual bool
diff --git a/dom/workers/WorkerScope.cpp b/dom/workers/WorkerScope.cpp
index 746ac3af9294..de6ffcc33715 100644
--- a/dom/workers/WorkerScope.cpp
+++ b/dom/workers/WorkerScope.cpp
@@ -599,9 +599,7 @@ public:
     RefPtr<SkipWaitingResultRunnable> runnable =
       new SkipWaitingResultRunnable(workerPrivate, mPromiseProxy);
 
-    AutoJSAPI jsapi;
-    jsapi.Init();
-    if (!runnable->Dispatch(jsapi.cx())) {
+    if (!runnable->Dispatch()) {
       NS_WARNING("Failed to dispatch SkipWaitingResultRunnable to the worker.");
     }
     return NS_OK;
diff --git a/dom/workers/XMLHttpRequest.cpp b/dom/workers/XMLHttpRequest.cpp
index 8ff06e0c7959..2a1b5e62575d 100644
--- a/dom/workers/XMLHttpRequest.cpp
+++ b/dom/workers/XMLHttpRequest.cpp
@@ -223,7 +223,7 @@ public:
   NS_DECL_ISUPPORTS_INHERITED
 
   bool
-  Dispatch(JSContext* aCx)
+  Dispatch()
   {
     mWorkerPrivate->AssertIsOnWorkerThread();
 
@@ -231,7 +231,6 @@ public:
     mSyncLoopTarget = syncLoop.EventTarget();
 
     if (NS_FAILED(NS_DispatchToMainThread(this))) {
-      JS_ReportError(aCx, "Failed to dispatch to main thread!");
       return false;
     }
 
@@ -595,10 +594,6 @@ public:
     mScopeObj(nsContentUtils::RootingCxForThread(), aScopeObj)
   { }
 
-  void Dispatch()
-  {
-    MainThreadProxyRunnable::Dispatch(nullptr);
-  }
 private:
   ~EventRunnable()
   { }
@@ -974,7 +969,7 @@ Proxy::Teardown(bool aSendUnpin)
       if (aSendUnpin) {
         RefPtr<XHRUnpinRunnable> runnable =
           new XHRUnpinRunnable(mWorkerPrivate, mXMLHttpRequestPrivate);
-        if (!runnable->Dispatch(nullptr)) {
+        if (!runnable->Dispatch()) {
           NS_RUNTIMEABORT("We're going to hang at shutdown anyways.");
         }
       }
@@ -985,7 +980,7 @@ Proxy::Teardown(bool aSendUnpin)
           new MainThreadStopSyncLoopRunnable(mWorkerPrivate,
                                              mSyncLoopTarget.forget(),
                                              false);
-        if (!runnable->Dispatch(nullptr)) {
+        if (!runnable->Dispatch()) {
           NS_RUNTIMEABORT("We're going to hang at shutdown anyways.");
         }
       }
@@ -1146,7 +1141,7 @@ LoadStartDetectionRunnable::Run()
       RefPtr<ProxyCompleteRunnable> runnable =
         new ProxyCompleteRunnable(mWorkerPrivate, mProxy,
                                   mXMLHttpRequestPrivate, mChannelId);
-      if (runnable->Dispatch(nullptr)) {
+      if (runnable->Dispatch()) {
         mProxy->mWorkerPrivate = nullptr;
         mProxy->mSyncLoopTarget = nullptr;
         mProxy->mOutstandingSendCount--;
@@ -1447,7 +1442,7 @@ WorkerThreadProxySyncRunnable::Run()
 
   RefPtr<ResponseRunnable> response =
     new ResponseRunnable(mWorkerPrivate, mProxy, rv);
-  if (!response->Dispatch(nullptr)) {
+  if (!response->Dispatch()) {
     MOZ_ASSERT(false, "Failed to dispatch response!");
   }
 
@@ -1692,7 +1687,7 @@ XMLHttpRequest::ReleaseProxy(ReleaseType aType)
         new SyncTeardownRunnable(mWorkerPrivate, mProxy);
       mProxy = nullptr;
 
-      if (!runnable->Dispatch(nullptr)) {
+      if (!runnable->Dispatch()) {
         NS_ERROR("Failed to dispatch teardown runnable!");
       }
     }
@@ -1869,12 +1864,10 @@ XMLHttpRequest::SendInternal(SendRunnable* aRunnable,
 
   mProxy->mOuterChannelId++;
 
-  JSContext* cx = mWorkerPrivate->GetJSContext();
-
   aRunnable->SetSyncLoopTarget(syncLoopTarget);
   aRunnable->SetHaveUploadListeners(hasUploadListeners);
 
-  if (!aRunnable->Dispatch(cx)) {
+  if (!aRunnable->Dispatch()) {
     // Dispatch() may have spun the event loop and we may have already unrooted.
     // If so we don't want autoUnpin to try again.
     if (!mRooted) {
@@ -1941,7 +1934,7 @@ XMLHttpRequest::Open(const nsACString& aMethod, const nsAString& aUrl,
                      mTimeout);
 
   ++mProxy->mOpenCount;
-  if (!runnable->Dispatch(mWorkerPrivate->GetJSContext())) {
+  if (!runnable->Dispatch()) {
     if (!--mProxy->mOpenCount) {
       ReleaseProxy();
     }
@@ -1978,7 +1971,7 @@ XMLHttpRequest::SetRequestHeader(const nsACString& aHeader,
 
   RefPtr<SetRequestHeaderRunnable> runnable =
     new SetRequestHeaderRunnable(mWorkerPrivate, mProxy, aHeader, aValue);
-  if (!runnable->Dispatch(mWorkerPrivate->GetJSContext())) {
+  if (!runnable->Dispatch()) {
     aRv.Throw(NS_ERROR_FAILURE);
     return;
   }
@@ -2004,7 +1997,7 @@ XMLHttpRequest::SetTimeout(uint32_t aTimeout, ErrorResult& aRv)
 
   RefPtr<SetTimeoutRunnable> runnable =
     new SetTimeoutRunnable(mWorkerPrivate, mProxy, aTimeout);
-  if (!runnable->Dispatch(mWorkerPrivate->GetJSContext())) {
+  if (!runnable->Dispatch()) {
     aRv.Throw(NS_ERROR_FAILURE);
     return;
   }
@@ -2030,7 +2023,7 @@ XMLHttpRequest::SetWithCredentials(bool aWithCredentials, ErrorResult& aRv)
 
   RefPtr<SetWithCredentialsRunnable> runnable =
     new SetWithCredentialsRunnable(mWorkerPrivate, mProxy, aWithCredentials);
-  if (!runnable->Dispatch(mWorkerPrivate->GetJSContext())) {
+  if (!runnable->Dispatch()) {
     aRv.Throw(NS_ERROR_FAILURE);
     return;
   }
@@ -2058,7 +2051,7 @@ XMLHttpRequest::SetMozBackgroundRequest(bool aBackgroundRequest,
   RefPtr<SetBackgroundRequestRunnable> runnable =
     new SetBackgroundRequestRunnable(mWorkerPrivate, mProxy,
                                      aBackgroundRequest);
-  if (!runnable->Dispatch(mWorkerPrivate->GetJSContext())) {
+  if (!runnable->Dispatch()) {
     aRv.Throw(NS_ERROR_FAILURE);
     return;
   }
@@ -2290,7 +2283,7 @@ XMLHttpRequest::Abort(ErrorResult& aRv)
   mProxy->mOuterEventStreamId++;
 
   RefPtr<AbortRunnable> runnable = new AbortRunnable(mWorkerPrivate, mProxy);
-  if (!runnable->Dispatch(mWorkerPrivate->GetJSContext())) {
+  if (!runnable->Dispatch()) {
     aRv.Throw(NS_ERROR_FAILURE);
     return;
   }
@@ -2316,7 +2309,7 @@ XMLHttpRequest::GetResponseHeader(const nsACString& aHeader,
   RefPtr<GetResponseHeaderRunnable> runnable =
     new GetResponseHeaderRunnable(mWorkerPrivate, mProxy, aHeader,
                                   responseHeader);
-  if (!runnable->Dispatch(mWorkerPrivate->GetJSContext())) {
+  if (!runnable->Dispatch()) {
     aRv.Throw(NS_ERROR_FAILURE);
     return;
   }
@@ -2342,7 +2335,7 @@ XMLHttpRequest::GetAllResponseHeaders(nsACString& aResponseHeaders,
   nsCString responseHeaders;
   RefPtr<GetAllResponseHeadersRunnable> runnable =
     new GetAllResponseHeadersRunnable(mWorkerPrivate, mProxy, responseHeaders);
-  if (!runnable->Dispatch(mWorkerPrivate->GetJSContext())) {
+  if (!runnable->Dispatch()) {
     aRv.Throw(NS_ERROR_FAILURE);
     return;
   }
@@ -2374,7 +2367,7 @@ XMLHttpRequest::OverrideMimeType(const nsAString& aMimeType, ErrorResult& aRv)
 
   RefPtr<OverrideMimeTypeRunnable> runnable =
     new OverrideMimeTypeRunnable(mWorkerPrivate, mProxy, aMimeType);
-  if (!runnable->Dispatch(mWorkerPrivate->GetJSContext())) {
+  if (!runnable->Dispatch()) {
     aRv.Throw(NS_ERROR_FAILURE);
     return;
   }
@@ -2409,7 +2402,7 @@ XMLHttpRequest::SetResponseType(XMLHttpRequestResponseType aResponseType,
 
   RefPtr<SetResponseTypeRunnable> runnable =
     new SetResponseTypeRunnable(mWorkerPrivate, mProxy, responseType);
-  if (!runnable->Dispatch(mWorkerPrivate->GetJSContext())) {
+  if (!runnable->Dispatch()) {
     aRv.Throw(NS_ERROR_FAILURE);
     return;
   }