content-policy.js

    // Non-matching domain or a resource directly loaded into a tab
    if (!isDenied (uri.host) || Ci.nsIContentPolicy.TYPE_DOCUMENT === typeCode) {
          return Ci.nsIContentPolicy.ACCEPT;
    }

    // ====================================
    //  console.log('resourceURL_leak_has_been_prevented');
    //          ^---    OR   ---v

    var bmessage = "ResourceLeak extension has prevented an attempt by web content to load one or more resource:// URIs";
    if (typeof window === "undefined"){
           var luwin = Cc["@mozilla.org/appshell/window-mediator;1"]
               .getService(Ci.nsIWindowMediator)
               .getMostRecentWindow("navigator:browser");
    }
    if (typeof gBrowser === "undefined"){
           var gBrowser = luwin.gBrowser;
    }
    let notif = gBrowser.getNotificationBox();
    let buttons = [];
    notif.appendNotification(bmessage, 'uniqID', null,
           notif.PRIORITY_WARNING_MEDIUM,  buttons   );
    // ====================================
    return Ci.nsIContentPolicy.REJECT_REQUEST;
  }