This is a simple, but somewhat awkward bug, on the games home page (when you not logged in).
The carousel controls are displayed as rectangles instead of being round. It isn't really a huge bug, but neither should it be a problem to fix.

The problem, if I got it right, is caused by the CSS being outdated. Some time ago Firefox dropped support for "-moz-border-radius", and began using "border-radius" instead.
So the solution would be to use both "-moz-border-radius" and "border-radius" in the CSS (to support both newer and older browsers).
It would probably also be worth using "-o-border-radius", "-ms-border-radius" and "-webkit-border-radius", to ensure it works in other browsers as well.