add check if no mention sound is set in Firefox sound notification workaround

js/interface_common.js

@@ -3083,7 +3083,8 @@ $(function () {
     // need to play something once in result of click on Firefox to be able to play sound notifications
     // FXIME it's just workaround of FF's bug, see https://bugzilla.mozilla.org/show_bug.cgi?id=1197631
     $('body').on('click', function () {
-        playSound('player', $.Options.sndMention.val, 0.001);
+        if ($.Options.sndMention.val !== 'false')
+            playSound('player', $.Options.sndMention.val, 0.001);
         setTimeout(function () {$('body').off('click');}, 10000);  // sound must be played before unbinding
     });
 });