improved bug fix for Firefox

improved workaround for bug 875114 in Firefox (more information: https://bugzilla.mozilla.org/show_bug.cgi?id=875114)
thanks to kkapsner (https://github.com/kkapsner) for his suggestion (http://forum.jswelt.de/tutorials-javascript/59128-beispiel-f-r-html5-webnotification-derzeit-nur-chrome-firefox.html#post377690)