diff --git a/dom/workers/XMLHttpRequest.cpp b/dom/workers/XMLHttpRequest.cpp
index a15fe20794b4..a241f6c0f0cf 100644
--- a/dom/workers/XMLHttpRequest.cpp
+++ b/dom/workers/XMLHttpRequest.cpp
@@ -564,6 +564,17 @@ private:
   MainThreadRun() MOZ_OVERRIDE
   {
     mProxy->Teardown();
+
+    // Make sure to stop the syncLoop of a sync XHR request whose
+    // worker was released while the request was still running.
+    if (mProxy->mSyncLoopTarget) {
+      nsRefPtr<MainThreadStopSyncLoopRunnable> runnable =
+        new MainThreadStopSyncLoopRunnable(mWorkerPrivate,
+                                           mProxy->mSyncLoopTarget.forget(),
+                                           false);
+      MOZ_ALWAYS_TRUE(runnable->Dispatch(nullptr));
+    }
+
     return NS_OK;
   }
 };
@@ -1108,6 +1119,9 @@ LoadStartDetectionRunnable::Run()
         mProxy->mWorkerPrivate = nullptr;
         mProxy->mOutstandingSendCount--;
       }
+    } else {
+      // The proxy was released and torn down by a SyncTeardownRunnable.
+      mProxy->mWorkerPrivate = nullptr;
     }
   }