diff --git a/browser/base/content/test/performance/browser_startup.js b/browser/base/content/test/performance/browser_startup.js
index 05a106933074..b7d328612567 100644
--- a/browser/base/content/test/performance/browser_startup.js
+++ b/browser/base/content/test/performance/browser_startup.js
@@ -49,9 +49,6 @@ const startupPhases = {
 
   // We are at this phase after creating the first browser window (ie. after final-ui-startup).
   "before opening first browser window": {blacklist: {
-    components: new Set([
-      "nsAsyncShutdown.js",
-    ]),
     modules: new Set([
       "resource://gre/modules/PlacesBackups.jsm",
       "resource://gre/modules/PlacesUtils.jsm",
diff --git a/dom/workers/ServiceWorkerRegistrar.cpp b/dom/workers/ServiceWorkerRegistrar.cpp
index 249aee540f75..f90bb735e064 100644
--- a/dom/workers/ServiceWorkerRegistrar.cpp
+++ b/dom/workers/ServiceWorkerRegistrar.cpp
@@ -54,7 +54,8 @@ StaticRefPtr<ServiceWorkerRegistrar> gServiceWorkerRegistrar;
 } // namespace
 
 NS_IMPL_ISUPPORTS(ServiceWorkerRegistrar,
-                  nsIObserver)
+                  nsIObserver,
+                  nsIAsyncShutdownBlocker)
 
 void
 ServiceWorkerRegistrar::Initialize()
@@ -73,10 +74,6 @@ ServiceWorkerRegistrar::Initialize()
     DebugOnly<nsresult> rv = obs->AddObserver(gServiceWorkerRegistrar,
                                               "profile-after-change", false);
     MOZ_ASSERT(NS_SUCCEEDED(rv));
-
-    rv = obs->AddObserver(gServiceWorkerRegistrar, "profile-before-change",
-                          false);
-    MOZ_ASSERT(NS_SUCCEEDED(rv));
   }
 }
 
@@ -94,7 +91,6 @@ ServiceWorkerRegistrar::ServiceWorkerRegistrar()
   : mMonitor("ServiceWorkerRegistrar.mMonitor")
   , mDataLoaded(false)
   , mShuttingDown(false)
-  , mShutdownCompleteFlag(nullptr)
   , mRunnableCounter(0)
 {
   MOZ_ASSERT(NS_IsMainThread());
@@ -829,8 +825,8 @@ ServiceWorkerRegistrar::ShutdownCompleted()
 {
   MOZ_ASSERT(NS_IsMainThread());
 
-  MOZ_ASSERT(mShutdownCompleteFlag && !*mShutdownCompleteFlag);
-  *mShutdownCompleteFlag = true;
+  DebugOnly<nsresult> rv = GetShutdownPhase()->RemoveBlocker(this);
+  MOZ_ASSERT(NS_SUCCEEDED(rv));
 }
 
 void
@@ -1027,6 +1023,13 @@ ServiceWorkerRegistrar::ProfileStarted()
     return;
   }
 
+  rv = GetShutdownPhase()->AddBlocker(
+    this, NS_LITERAL_STRING(__FILE__), __LINE__,
+    NS_LITERAL_STRING("ServiceWorkerRegistrar: Flushing data"));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return;
+  }
+
   nsCOMPtr<nsIEventTarget> target =
     do_GetService(NS_STREAMTRANSPORTSERVICE_CONTRACTID);
   MOZ_ASSERT(target, "Must have stream transport service");
@@ -1061,12 +1064,41 @@ ServiceWorkerRegistrar::ProfileStopped()
     return;
   }
 
-  bool completed = false;
-  mShutdownCompleteFlag = &completed;
-
   child->SendShutdownServiceWorkerRegistrar();
+}
 
-  MOZ_ALWAYS_TRUE(SpinEventLoopUntil([&]() { return completed; }));
+// Async shutdown blocker methods
+
+NS_IMETHODIMP
+ServiceWorkerRegistrar::BlockShutdown(nsIAsyncShutdownClient* aClient)
+{
+  ProfileStopped();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+ServiceWorkerRegistrar::GetName(nsAString& aName)
+{
+  aName = NS_LITERAL_STRING("ServiceWorkerRegistrar: Flushing data");
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+ServiceWorkerRegistrar::GetState(nsIPropertyBag**)
+{
+  return NS_OK;
+}
+
+nsCOMPtr<nsIAsyncShutdownClient>
+ServiceWorkerRegistrar::GetShutdownPhase() const
+{
+  nsCOMPtr<nsIAsyncShutdownService> svc = services::GetAsyncShutdown();
+  MOZ_RELEASE_ASSERT(svc);
+
+  nsCOMPtr<nsIAsyncShutdownClient> client;
+  Unused << svc->GetProfileBeforeChange(getter_AddRefs(client));
+  MOZ_RELEASE_ASSERT(client);
+  return Move(client);
 }
 
 void
@@ -1097,17 +1129,6 @@ ServiceWorkerRegistrar::Observe(nsISupports* aSubject, const char* aTopic,
     return NS_OK;
   }
 
-  if (!strcmp(aTopic, "profile-before-change")) {
-    nsCOMPtr<nsIObserverService> observerService =
-      services::GetObserverService();
-    observerService->RemoveObserver(this, "profile-before-change");
-
-    // Shutting down, let's sync the data.
-    ProfileStopped();
-
-    return NS_OK;
-  }
-
   MOZ_ASSERT(false, "ServiceWorkerRegistrar got unexpected topic!");
   return NS_ERROR_UNEXPECTED;
 }
diff --git a/dom/workers/ServiceWorkerRegistrar.h b/dom/workers/ServiceWorkerRegistrar.h
index e90fe32fafb7..be7452ce0fa1 100644
--- a/dom/workers/ServiceWorkerRegistrar.h
+++ b/dom/workers/ServiceWorkerRegistrar.h
@@ -10,6 +10,7 @@
 #include "mozilla/Monitor.h"
 #include "mozilla/Telemetry.h"
 #include "nsClassHashtable.h"
+#include "nsIAsyncShutdown.h"
 #include "nsIObserver.h"
 #include "nsCOMPtr.h"
 #include "nsString.h"
@@ -34,12 +35,14 @@ namespace dom {
 class ServiceWorkerRegistrationData;
 
 class ServiceWorkerRegistrar : public nsIObserver
+                             , public nsIAsyncShutdownBlocker
 {
   friend class ServiceWorkerRegistrarSaveDataRunnable;
 
 public:
   NS_DECL_THREADSAFE_ISUPPORTS
   NS_DECL_NSIOBSERVER
+  NS_DECL_NSIASYNCSHUTDOWNBLOCKER
 
   static void Initialize();
 
@@ -79,6 +82,8 @@ private:
   void ShutdownCompleted();
   void MaybeScheduleShutdownCompleted();
 
+  nsCOMPtr<nsIAsyncShutdownClient> GetShutdownPhase() const;
+
   bool IsSupportedVersion(const nsACString& aVersion) const;
 
   mozilla::Monitor mMonitor;
@@ -91,7 +96,6 @@ protected:
 
   // PBackground thread only
   bool mShuttingDown;
-  bool* mShutdownCompleteFlag;
   uint32_t mRunnableCounter;
 };