I ran into a similar issue with the overlay not showing in FF, to fix I changed the following in ibox.js from:
else els.overlay.style.backgroundImage = "url('" + _pub.base_url + "images/bg.png')";
to:
else els.overlay.style.backgroundImage = "url('/images/bg.png')";
Works like a charm, assuming you place bg.png in the image directory and that directory is in the site's root...