[mappu]

It looks fine to me ( http://i.imgur.com/rAYGIwX.png ).

What fork of firefox is that? I don't recognise the taskbar icon. Does the problem still happen with upstream firefox?

[JohnTHaller]

Tor Browser, likely. It's based on the older ESR branch of Firefox. As it's a modified version of Firefox, it's unsupported by Mozilla.