Internet Explorer and Firefox look at borders a bit differently. I am not sure if this is your problem, but it affects sites and can affect them greatly depending how your site is set up.
IE, I believe does not count the border in the length of the table (or other object such as divs). So if the table is 800px wide and there is a 2px border, then the width is actually 804px. In Firefox it would be 800px no matter what.
I hope I stated that accurately 
kw