it seems that the loader finished its work slower in firefox (half a second slower? maybe it has something to do with loading-line fading out), and missed the chance to remove the qLtempOverlay overlay div.
onComplete: function(){
//fix firefox bug
if(navigator.userAgent.indexOf('Firefox') !== -1){
$('#qLtempOverlay').remove();
}
}
tested on firefox 42/43
ps, in my case, the queryloader2.min.js script is loaded via $.getScript().