--- a/dom/workers/ServiceWorkerManager.cpp
+++ b/dom/workers/ServiceWorkerManager.cpp
@@ -869,17 +869,25 @@ public:
 
       if (mRegistration) {
         nsRefPtr<ServiceWorkerInfo> newest = mRegistration->Newest();
         if (newest && mScriptSpec.Equals(newest->ScriptSpec()) &&
             mScriptSpec.Equals(mRegistration->mScriptSpec)) {
           mRegistration->mPendingUninstall = false;
           swm->StoreRegistration(mPrincipal, mRegistration);
           Succeed();
-          Done(NS_OK);
+
+          // Done() must always be called async from Start()
+          nsCOMPtr<nsIRunnable> runnable =
+            NS_NewRunnableMethodWithArg<nsresult>(
+              this,
+              &ServiceWorkerRegisterJob::Done,
+              NS_OK);
+          MOZ_ALWAYS_TRUE(NS_SUCCEEDED(NS_DispatchToCurrentThread(runnable)));
+
           return;
         }
       } else {
         mRegistration = swm->CreateNewRegistration(mScope, mPrincipal);
       }
 
       mRegistration->mScriptSpec = mScriptSpec;
       swm->StoreRegistration(mPrincipal, mRegistration);