Something weird is happening in FF (I am running 5 at the moment, but it also happens in 4) for this site. It is adding about 50px to the top of the page and I cannot see what element it is being applied to. I cannot duplicate this issue in any other browser.
Well one thing you could try is to validate your css and html, your css showed 6 errors and your html 4 errors. Sometimes these errors can have weird and wonderfully annoying effects. If that doesn’t work then I’m sure someone else will be along shortly to help figure it out. Also, Since you use firefox, there is a plugin named web developer 1.1.9 that has a lot of usefull quick access features that you might want to check out like validators and displaying outlines on your block elements, turning on and off different pieces of css etc.