--- a/toolkit/components/build/nsToolkitCompsModule.cpp
+++ b/toolkit/components/build/nsToolkitCompsModule.cpp
@@ -115,17 +115,17 @@ nsUrlClassifierDBServiceConstructor(nsIS
     return rv;
 }
 #endif
 
 NS_GENERIC_FACTORY_CONSTRUCTOR(nsBrowserStatusFilter)
 #if defined(MOZ_UPDATER) && !defined(MOZ_WIDGET_ANDROID)
 NS_GENERIC_FACTORY_CONSTRUCTOR(nsUpdateProcessor)
 #endif
-NS_GENERIC_FACTORY_CONSTRUCTOR(FinalizationWitnessService)
+NS_GENERIC_FACTORY_CONSTRUCTOR_INIT(FinalizationWitnessService, Init)
 NS_GENERIC_FACTORY_CONSTRUCTOR(NativeOSFileInternalsService)
 NS_GENERIC_FACTORY_CONSTRUCTOR_INIT(NativeFileWatcherService, Init)
 
 NS_GENERIC_FACTORY_SINGLETON_CONSTRUCTOR(AddonPathService, AddonPathService::GetInstance)
 
 NS_DEFINE_NAMED_CID(NS_TOOLKIT_APPSTARTUP_CID);
 #if defined(MOZ_HAS_PERFSTATS)
 NS_DEFINE_NAMED_CID(NS_TOOLKIT_PERFORMANCESTATSSERVICE_CID);

--- a/toolkit/components/finalizationwitness/FinalizationWitnessService.cpp
+++ b/toolkit/components/finalizationwitness/FinalizationWitnessService.cpp
@@ -13,16 +13,18 @@
 #include "mozilla/Scoped.h"
 #include "mozilla/Services.h"
 #include "nsIObserverService.h"
 #include "nsThreadUtils.h"
 
 
 // Implementation of nsIFinalizationWitnessService
 
+static bool gShuttingDown = false;
+
 namespace mozilla {
 
 namespace {
 
 /**
  * An event meant to be dispatched to the main thread upon finalization
  * of a FinalizationWitness, unless method |forget()| has been called.
  *
@@ -94,18 +96,18 @@ ExtractFinalizationEvent(JSObject *objSe
  * Finalizer for instances of FinalizationWitness.
  *
  * Unless method Forget() has been called, the finalizer displays an error
  * message.
  */
 void Finalize(JSFreeOp *fop, JSObject *objSelf)
 {
   nsRefPtr<FinalizationEvent> event = ExtractFinalizationEvent(objSelf);
-  if (event == nullptr) {
-    // Forget() has been called
+  if (event == nullptr || gShuttingDown) {
+    // NB: event will be null if Forget() has been called
     return;
   }
 
   // Notify observers. Since we are executed during garbage-collection,
   // we need to dispatch the notification to the main thread.
   (void)NS_DispatchToMainThread(event);
   // We may fail at dispatching to the main thread if we arrive too late
   // during shutdown. In that case, there is not much we can do.
@@ -166,17 +168,17 @@ bool Forget(JSContext *cx, unsigned argc
 
 static const JSFunctionSpec sWitnessClassFunctions[] = {
   JS_FN("forget", Forget, 0, JSPROP_READONLY | JSPROP_PERMANENT),
   JS_FS_END
 };
 
 }
 
-NS_IMPL_ISUPPORTS(FinalizationWitnessService, nsIFinalizationWitnessService)
+NS_IMPL_ISUPPORTS(FinalizationWitnessService, nsIFinalizationWitnessService, nsIObserver)
 
 /**
  * Create a new Finalization Witness.
  *
  * A finalization witness is an object whose sole role is to notify
  * observers when it is gc-ed. Once the witness is created, call its
  * method |forget()| to prevent the observers from being notified.
  *
@@ -204,9 +206,35 @@ FinalizationWitnessService::Make(const c
   // Transfer ownership of the addrefed |event| to |objResult|.
   JS_SetReservedSlot(objResult, WITNESS_SLOT_EVENT,
                      JS::PrivateValue(event.forget().take()));
 
   aRetval.setObject(*objResult);
   return NS_OK;
 }
 
+NS_IMETHODIMP
+FinalizationWitnessService::Observe(nsISupports* aSubject,
+                                    const char* aTopic,
+                                    const char16_t* aValue)
+{
+  MOZ_ASSERT(strcmp(aTopic, NS_XPCOM_SHUTDOWN_OBSERVER_ID) == 0);
+  gShuttingDown = true;
+  nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
+  if (obs) {
+    obs->RemoveObserver(this, NS_XPCOM_SHUTDOWN_OBSERVER_ID);
+  }
+
+  return NS_OK;
+}
+
+nsresult
+FinalizationWitnessService::Init()
+{
+  nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
+  if (!obs) {
+    return NS_ERROR_FAILURE;
+  }
+
+  return obs->AddObserver(this, NS_XPCOM_SHUTDOWN_OBSERVER_ID, false);
+}
+
 } // namespace mozilla

--- a/toolkit/components/finalizationwitness/FinalizationWitnessService.h
+++ b/toolkit/components/finalizationwitness/FinalizationWitnessService.h
@@ -1,27 +1,32 @@
 /* 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/. */
 
 #ifndef mozilla_finalizationwitnessservice_h__
 #define mozilla_finalizationwitnessservice_h__
 
 #include "nsIFinalizationWitnessService.h"
+#include "nsIObserver.h"
 
 namespace mozilla {
 
 /**
  * XPConnect initializer, for use in the main thread.
  */
-class FinalizationWitnessService final : public nsIFinalizationWitnessService
+class FinalizationWitnessService final : public nsIFinalizationWitnessService,
+                                         public nsIObserver
 {
  public:
   NS_DECL_ISUPPORTS
   NS_DECL_NSIFINALIZATIONWITNESSSERVICE
+  NS_DECL_NSIOBSERVER
+
+  nsresult Init();
  private:
   ~FinalizationWitnessService() {}
   void operator=(const FinalizationWitnessService* other) = delete;
 };
 
 } // namespace mozilla
 
 #endif // mozilla_finalizationwitnessservice_h__