Strange table-width problem, css related probably

This is how it looks like sometimes in FF only. I have the punbb css, AND an own one, but I dont know where to look for the problem? What could cause this?

Btw, the example is from PM-mod, but the problem is in the forum index also.

If i hit the reload button after it looks like this it's showed correctly, this happens only sometimes when i enter the page through a link.