I use only FF and i see all things right. Actually i think i know where the problem lies... Take your mouse cursor on the column where "Categories" is written...in the left sidebar. It's like knocking on the door and Shazaam ...the categories appear
Yep that works, not good design though, there's no reason to place your cursor on the word 'category' which you need to do to see the categories, it should display automatically for the benefit of users and webmaster.