diff --git a/platform/firefox/frameScript.js b/platform/firefox/frameScript.js
index c97e9ac..560eca2 100644
--- a/platform/firefox/frameScript.js
+++ b/platform/firefox/frameScript.js
@@ -19,6 +19,8 @@
     Home: https://github.com/gorhill/uBlock
 */
 
+'use strict';
+
 /******************************************************************************/
 
 Components.utils.import(