--- a/gfx/layers/apz/public/GeckoContentController.h
+++ b/gfx/layers/apz/public/GeckoContentController.h
@@ -82,16 +82,17 @@ public:
    */
   virtual void SendAsyncScrollDOMEvent(bool aIsRoot,
                                        const CSSRect &aContentRect,
                                        const CSSSize &aScrollableSize) = 0;
 
   /**
    * Schedules a runnable to run on the controller/UI thread at some time
    * in the future.
+   * This method must always be called on the controller thread.
    */
   virtual void PostDelayedTask(Task* aTask, int aDelayMs) = 0;
 
   /**
    * Retrieves the last known zoom constraints for the root scrollable layer
    * for this layers tree. This function should return false if there are no
    * last known zoom constraints.
    */

--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -3133,16 +3133,17 @@ void AsyncPanZoomController::UpdateZoomC
 ZoomConstraints
 AsyncPanZoomController::GetZoomConstraints() const
 {
   return mZoomConstraints;
 }
 
 
 void AsyncPanZoomController::PostDelayedTask(Task* aTask, int aDelayMs) {
+  AssertOnControllerThread();
   nsRefPtr<GeckoContentController> controller = GetGeckoContentController();
   if (controller) {
     controller->PostDelayedTask(aTask, aDelayMs);
   }
 }
 
 void AsyncPanZoomController::SendAsyncScrollEvent() {
   nsRefPtr<GeckoContentController> controller = GetGeckoContentController();