diff --git a/dom/base/WebSocket.cpp b/dom/base/WebSocket.cpp
index 1ae6ebf2f1b..418afea439f 100644
--- a/dom/base/WebSocket.cpp
+++ b/dom/base/WebSocket.cpp
@@ -2523,6 +2523,9 @@ public:
   bool
   PreDispatch(JSContext* aCx, WorkerPrivate* aWorkerPrivate)
   {
+    // We don't call WorkerRunnable::PreDispatch because it would assert the
+    // wrong thing about which thread we're on.
+    AssertIsOnMainThread();
     return true;
   }
 
@@ -2530,6 +2533,9 @@ public:
   PostDispatch(JSContext* aCx, WorkerPrivate* aWorkerPrivate,
                bool aDispatchResult)
   {
+    // We don't call WorkerRunnable::PostDispatch because it would assert the
+    // wrong thing about which thread we're on.
+    AssertIsOnMainThread();
   }
 
 private:
@@ -2696,6 +2702,10 @@ public:
   bool
   PreDispatch(JSContext* aCx, WorkerPrivate* aWorkerPrivate)
   {
+    // We don't call WorkerRunnable::PreDispatch because it would assert the
+    // wrong thing about which thread we're on.  We're on whichever thread the
+    // channel implementation is running on (probably the main thread or socket
+    // transport thread).
     return true;
   }
 
@@ -2703,6 +2713,10 @@ public:
   PostDispatch(JSContext* aCx, WorkerPrivate* aWorkerPrivate,
                bool aDispatchResult)
   {
+    // We don't call WorkerRunnable::PreDispatch because it would assert the
+    // wrong thing about which thread we're on.  We're on whichever thread the
+    // channel implementation is running on (probably the main thread or socket
+    // transport thread).
   }
 
 private:
diff --git a/dom/notification/Notification.cpp b/dom/notification/Notification.cpp
index e68e510c7af..b81b387aa0f 100644
--- a/dom/notification/Notification.cpp
+++ b/dom/notification/Notification.cpp
@@ -375,6 +375,9 @@ protected:
   bool
   PreDispatch(JSContext* aCx, WorkerPrivate* aWorkerPrivate) override
   {
+    // We don't call WorkerRunnable::PreDispatch because it would assert the
+    // wrong thing about which thread we're on.
+    AssertIsOnMainThread();
     return true;
   }
 
@@ -382,6 +385,9 @@ protected:
   PostDispatch(JSContext* aCx, WorkerPrivate* aWorkerPrivate,
                bool aDispatchResult) override
   {
+    // We don't call WorkerRunnable::PostDispatch because it would assert the
+    // wrong thing about which thread we're on.
+    AssertIsOnMainThread();
   }
 
   bool
diff --git a/dom/workers/ServiceWorkerPrivate.cpp b/dom/workers/ServiceWorkerPrivate.cpp
index 54a169bac80..578c985903c 100644
--- a/dom/workers/ServiceWorkerPrivate.cpp
+++ b/dom/workers/ServiceWorkerPrivate.cpp
@@ -319,6 +319,8 @@ public:
     nsCOMPtr<nsIRunnable> runnable =
       new RegistrationUpdateRunnable(mRegistration, true /* time check */);
     NS_DispatchToMainThread(runnable.forget());
+
+    ExtendableEventWorkerRunnable::PostRun(aCx, aWorkerPrivate, aRunResult);
   }
 };
 
diff --git a/dom/workers/WorkerPrivate.cpp b/dom/workers/WorkerPrivate.cpp
index 848de8e9c80..151c5270694 100644
--- a/dom/workers/WorkerPrivate.cpp
+++ b/dom/workers/WorkerPrivate.cpp
@@ -610,9 +610,7 @@ private:
     WorkerRunnable::PostRun(aCx, aWorkerPrivate, aRunResult);
 
     // Match the busy count increase from NotifyRunnable.
-    if (!aWorkerPrivate->ModifyBusyCountFromWorker(aCx, false)) {
-      JS_ReportPendingException(aCx);
-    }
+    aWorkerPrivate->ModifyBusyCountFromWorker(aCx, false);
 
     aWorkerPrivate->CloseHandlerFinished();
   }
diff --git a/dom/workers/WorkerRunnable.cpp b/dom/workers/WorkerRunnable.cpp
index 416f7eb46a2..952a4d83f89 100644
--- a/dom/workers/WorkerRunnable.cpp
+++ b/dom/workers/WorkerRunnable.cpp
@@ -223,9 +223,7 @@ WorkerRunnable::PostRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate,
 #endif
 
   if (mBehavior == WorkerThreadModifyBusyCount) {
-    if (!aWorkerPrivate->ModifyBusyCountFromWorker(aCx, false)) {
-      aRunResult = false;
-    }
+    aWorkerPrivate->ModifyBusyCountFromWorker(aCx, false);
   }
 
   if (!aRunResult) {
@@ -644,6 +642,9 @@ bool
 WorkerSameThreadRunnable::PreDispatch(JSContext* aCx,
                                       WorkerPrivate* aWorkerPrivate)
 {
+  // We don't call WorkerRunnable::PreDispatch, because we're using
+  // WorkerThreadModifyBusyCount for mBehavior, and WorkerRunnable will assert
+  // that PreDispatch is on the parent thread in that case.
   aWorkerPrivate->AssertIsOnWorkerThread();
   return true;
 }
@@ -653,6 +654,9 @@ WorkerSameThreadRunnable::PostDispatch(JSContext* aCx,
                                        WorkerPrivate* aWorkerPrivate,
                                        bool aDispatchResult)
 {
+  // We don't call WorkerRunnable::PostDispatch, because we're using
+  // WorkerThreadModifyBusyCount for mBehavior, and WorkerRunnable will assert
+  // that PostDispatch is on the parent thread in that case.
   aWorkerPrivate->AssertIsOnWorkerThread();
   if (aDispatchResult) {
     DebugOnly<bool> willIncrement = aWorkerPrivate->ModifyBusyCountFromWorker(aCx, true);
@@ -661,21 +665,3 @@ WorkerSameThreadRunnable::PostDispatch(JSContext* aCx,
     MOZ_ASSERT(willIncrement);
   }
 }
-
-void
-WorkerSameThreadRunnable::PostRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate,
-                                  bool aRunResult)
-{
-  MOZ_ASSERT(aCx);
-  MOZ_ASSERT(aWorkerPrivate);
-
-  aWorkerPrivate->AssertIsOnWorkerThread();
-
-  DebugOnly<bool> willDecrement = aWorkerPrivate->ModifyBusyCountFromWorker(aCx, false);
-  MOZ_ASSERT(willDecrement);
-
-  if (!aRunResult) {
-    JS_ReportPendingException(aCx);
-  }
-}
-
diff --git a/dom/workers/WorkerRunnable.h b/dom/workers/WorkerRunnable.h
index a26181634de..62bfb7a051f 100644
--- a/dom/workers/WorkerRunnable.h
+++ b/dom/workers/WorkerRunnable.h
@@ -405,9 +405,8 @@ protected:
   PostDispatch(JSContext* aCx, WorkerPrivate* aWorkerPrivate,
                bool aDispatchResult) override;
 
-  virtual void
-  PostRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate,
-          bool aRunResult) override;
+  // We just delegate PostRun to WorkerRunnable, since it does exactly
+  // what we want.
 };
 
 // Base class for the runnable objects, which makes a synchronous call to