diff --git a/xpcom/glue/nsCycleCollectionParticipant.h b/xpcom/glue/nsCycleCollectionParticipant.h
index 86b5e599a73a..9fa4c4579ea4 100644
--- a/xpcom/glue/nsCycleCollectionParticipant.h
+++ b/xpcom/glue/nsCycleCollectionParticipant.h
@@ -481,7 +481,11 @@ public:
 #define NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(_field)              \
   NS_IMPL_CYCLE_COLLECTION_TRACE_JS_CALLBACK(tmp->_field)
 
+// NB: The (void)tmp; hack in the TRACE_END macro exists to support
+// implementations that don't need to do anything in their Trace method.
+// Without this hack, some compilers warn about the unused tmp local.
 #define NS_IMPL_CYCLE_COLLECTION_TRACE_END                                     \
+      (void)tmp;                                                               \
   }
 
 ///////////////////////////////////////////////////////////////////////////////