Few things to point out. Firstly, your page doesn't validate, so either reomve the validated link at the bottom or sort out the issues. Secondly, have you looked at it in IE7? The links across the top don't work as you presumably intended (like they do in Firefox) and the left menu section is in completely the wrong place!
Have you tried CSSVista
? That will allow you to see the site in both IE and Firefox so you can play about with the CSS and, in real-time, see the changes you make. I'm thinking that the problem you have has something to do with widths because your #content and #sidebar are actually overlapping, which works fine in Firefox but could be causing the issues in IE