Here’s the link Brian is referring to, but it may not help that much as you don’t have a Doctype declaration on your document:
Seems to be a Firefox only problem, so I would look at validating your code at www.w3.org. Frontpage is notorious for creating poor code, and FF is known for applying code validations more precisely than IE. If you validate the code, it should point to the problem you are having. I do agree with everyone else though, that it is probably in your mixed width and height tags.
Good luck!
Cheers