diff --git a/js/src/vm/MutexIDs.h b/js/src/vm/MutexIDs.h
index 7cf67634d731..b7922f8e1d74 100644
--- a/js/src/vm/MutexIDs.h
+++ b/js/src/vm/MutexIDs.h
@@ -12,7 +12,8 @@
 // Central definition point for mutex ordering.
 //
 // Mutexes can only be acquired in increasing order. This prevents the
-// possibility of deadlock.
+// possibility of deadlock. Mutexes with the same order cannot be held
+// at the same time.
 
 #define FOR_EACH_MUTEX(_)             \
   _(TestMutex, 100)                   \