In /sources/css/moz/common.css, you set margin-top for #nav to -84px, but in /sources/css/ie/common.css you have an extra margin-top set to 2px. Also, the top property is more appropriate than margin-top.
By the way, you might want to rethink your cross-browser style strategy. For one, creating a complete set of style sheets for each browser is much more work than writing a single set for all and individual sheets to fix browser bugs. Second, having a separate set for each browser means your site will be limited to browser default styling for any browser you don't target. Take a look at your site in Safari, Chrome or Opera.