1. Instead of adding the
border attributes in Body add it in the Page Background.
Leave the
margin attributes in Body. Remove "750px". Then go to StyleVars and add the "750" in the first row "Main Table Width (pixels or percentage)". *Do not add "px" after it.
2. IE and FF renders differently. To compensate, if you haven't already, create your gradient with the height of 45px. When .cat stretches then, it should have a gradual look in both browsers.
3. In Admin CP --> vBulletin Options --> Forum Display Options (forumdisplay) find "Hot Threads Enabled" and select "No".
See if these work.