Backed out changeset 689690a17de3 (bug 833143) since the attempt at merging the backout from m-c was lost.

toolkit/xre/nsXREDirProvider.cpp

@@ -847,6 +847,19 @@ nsXREDirProvider::DoShutdown()
       obsSvc->NotifyObservers(nullptr, "profile-change-net-teardown", kShutdownPersist);
       obsSvc->NotifyObservers(nullptr, "profile-change-teardown", kShutdownPersist);
 
+      // Phase 2c: Now that things are torn down, force JS GC so that things which depend on
+      // resources which are about to go away in "profile-before-change" are destroyed first.
+
+      nsCOMPtr<nsIJSRuntimeService> rtsvc
+        (do_GetService("@mozilla.org/js/xpc/RuntimeService;1"));
+      if (rtsvc)
+      {
+        JSRuntime *rt = nullptr;
+        rtsvc->GetRuntime(&rt);
+        if (rt)
+          ::JS_GC(rt);
+      }
+
       // Phase 3: Notify observers of a profile change
       obsSvc->NotifyObservers(nullptr, "profile-before-change", kShutdownPersist);
     }