diff --git a/xpcom/threads/nsThreadManager.cpp b/xpcom/threads/nsThreadManager.cpp
index 2f074d84c108..fb262188934d 100644
--- a/xpcom/threads/nsThreadManager.cpp
+++ b/xpcom/threads/nsThreadManager.cpp
@@ -191,6 +191,7 @@ BackgroundEventTarget::UnregisterShutdownTask(nsITargetShutdownTask* aTask) {
 
 void BackgroundEventTarget::BeginShutdown(
     nsTArray<RefPtr<ShutdownPromise>>& promises) {
+  MutexAutoLock lock(mMutex);
   for (auto& queue : mTaskQueues) {
     promises.AppendElement(queue->BeginShutdown());
   }