In IE, fine, but firefox (and I have had similar problems with other pages:
What could be causing this, and how could I fix the problem?
Can you provide a link to the exact page that has these problems.
Firefox and IE do render a number of elements differently. Mostly slightly different default margins and both browsers recognize certain tags the other doesn't.
My best guess is that you are using relative font size css directives and these are showing up larger in firefox then IE which causes them to wrap and overlay on the next absolutely positioned container. Try changing all font-sizes to pixels (eg font-size:10px, font-size:12px, etc...). Also consider setting the margin tops and bottoms manually for certain block level elements (like br, body, and h1-h6). I'm not sure but some of your fonts might be getting you into trouble as well. You probably want to stick with the name brand fonts (arial, times new roman, verdana, georgia, etc...).
I couldn't help you more without seeing an actual page and going over the source code.
Upgrading Firefox 2 to Firefox 3 shouldn't make a difference.
Like the site BTW...