diff --git a/xpcom/threads/LazyIdleThread.h b/xpcom/threads/LazyIdleThread.h
index 91336bde1932..56da00a6194b 100644
--- a/xpcom/threads/LazyIdleThread.h
+++ b/xpcom/threads/LazyIdleThread.h
@@ -50,6 +50,7 @@
 
 #include "mozilla/Mutex.h"
 #include "nsCOMPtr.h"
+#include "nsTArray.h"
 
 #define IDLE_THREAD_TOPIC "thread-shutting-down"