--- a/dom/media/MemoryBlockCache.cpp
+++ b/dom/media/MemoryBlockCache.cpp
@@ -66,20 +66,19 @@ private:
 
 NS_IMPL_ISUPPORTS(MemoryBlockCacheTelemetry,
                   nsIObserver,
                   nsISupportsWeakReference)
 
 /* static */ size_t
 MemoryBlockCacheTelemetry::NotifyCombinedSizeGrown(size_t aNewSize)
 {
-  NS_ASSERTION(NS_IsMainThread(), "Only call on main thread");
-
   // Ensure gMemoryBlockCacheTelemetry exists.
   if (!gMemoryBlockCacheTelemetry) {
+    MOZ_ASSERT(NS_IsMainThread());
     gMemoryBlockCacheTelemetry = new MemoryBlockCacheTelemetry();
 
     nsCOMPtr<nsIObserverService> observerService =
       mozilla::services::GetObserverService();
     if (observerService) {
       observerService->AddObserver(
         gMemoryBlockCacheTelemetry, "profile-change-teardown", true);
     }