No problem here with Firefox 3.0.10 on Vista. What are you using?
Rendering issue

Does this happen every time? Does it happen on any other websites?
This link http://forums.mozillazine.org/viewtopic.php?f=25&t=1928579
suggests the fault gradually starts happening and can be (temporarily) resolved by clearing the cache.
Has it started occurring after the chess.com upgrade - or after a Firefox upgrade?

for some reason linux is doing weird things with fonts. i recommend you get into your browser settings and make the font size a little smaller. very odd - not sure why linux does that (mac and pc are fine)

for some reason linux is doing weird things with fonts. i recommend you get into your browser settings and make the font size a little smaller. very odd - not sure why linux does that (mac and pc are fine)
Thank you everyone, but erik had the solution (I really should have guessed such a thing). I set the font size smaller and the navigation bar is clean.
http://img101.imageshack.us/img101/5986/screenshoteo.png