--- a/xpcom/threads/TimerThread.cpp
+++ b/xpcom/threads/TimerThread.cpp
@@ -265,17 +265,20 @@ nsTimerEvent::DeleteAllocatorIfNeeded()
     delete sAllocator;
     sAllocator = nullptr;
   }
 }
 
 NS_IMETHODIMP
 nsTimerEvent::Run()
 {
-  MOZ_ASSERT(mTimer);
+  if (!mTimer) {
+    MOZ_ASSERT(false);
+    return NS_OK;
+  }
 
   if (mGeneration != mTimer->GetGeneration()) {
     return NS_OK;
   }
 
   if (MOZ_LOG_TEST(GetTimerLog(), LogLevel::Debug)) {
     TimeStamp now = TimeStamp::Now();
     MOZ_LOG(GetTimerLog(), LogLevel::Debug,