It's also messed up in Opera, Safari and Chrome here.... Which leads me to the conclusion if it is working in FF, it is doing so due to incorrect behaviors, not correct ones.
Rayzur is correct in pointing out your validation errors, unclosed tags/invalid markup are the leading cause of broken layouts... With 77 validation errors that's not even HTML, it's gibberish.
... and that's before we talk fixed metric (aka px) fonts on the content, line-breaks doing padding's job...
You've got a lot to fix in the markup ALONE before worrying about layout.