Hi,
See, the problem is you designed using IE which is the least standards complaint browser so you end up with IE specific stuff in the main css that breaks more standards compliant browsers like Firefox.
I (and many others) recommend design first in Firefox because it is the most standards compliant then you can fix IE using IE specific stylesheets that don't affect other browsers.
http://www.mojoportal.com/understandingthecssfiles.aspx
I don't know a way to fix it going the other way because its hard to know what parts of your css are really IE specific.
Hope it helps,
Joe