CSS – CSS Reset
There are many hacks to get past cross-browser compatibility problems. I've run into way too many of them, even though I regularly check sites in both IE and FF. One of the reasons for the problems is that some browsers add padding and margin sizes to certain elements. To remove this Yahoo has a utility called Reset CSS. IT removes all added borders, margins, everything to level the playing field so you can precisely define how you want elements defined without anything added by a browser.
UPDATE: After reading some of the comments and advise on resetting, a lot of people have highly recommended Eric Meyer's CSS Reset stylesheet.

