CSS Problems (?) In FireFox

It might be a CSS problem, but it could also be the unclosed tags and other XHTML errors.

Validate your HTML:
http://validator.w3.org/ and correct the errors.

Then you can validate your CSS: http://jigsaw.w3.org/css-validator/

These tools are useful for pinpointing problems in your code. If they give you the all-clear and you still have a problem with your site, then at least you know the problem is not in the HTML or CSS per se.

BTW: There is an excellent (free) extension to Firefox called Web Developer:

http://chrispederick.com/work/webdeveloper/

It provides a menu bar with useful tools for trouble-shooting web pages, including direct links to the validator pages.