diff --git a/dom/system/OSFileConstants.h b/dom/system/OSFileConstants.h
index 918d8c15fc19..173bd3a9865d 100644
--- a/dom/system/OSFileConstants.h
+++ b/dom/system/OSFileConstants.h
@@ -7,6 +7,7 @@
 
 #include "jspubtd.h"
 #include "nsIOSFileConstantsService.h"
+#include "mozilla/Attributes.h"
 
 namespace mozilla {
 
@@ -43,7 +44,7 @@ bool DefineOSFileConstants(JSContext *cx, JSObject *global);
 /**
  * XPConnect initializer, for use in the main thread.
  */
-class OSFileConstantsService: public nsIOSFileConstantsService
+class OSFileConstantsService MOZ_FINAL : public nsIOSFileConstantsService
 {
  public:
   NS_DECL_ISUPPORTS