Although I'm a big fan of Firefox, I have to tell you something different:
If you want well-rendered sites, you have to make your site look good in an Acid2-compliant browser (OPERA 9)(which renders css absolutely as defined in the specs), and something which can handle Js as needed (firefox is the most advanced, opera is the fastest), and then create css and js files which are only used when the user agent is firefox or ie or anything. You can make this from PHP with the $_SERVER['HTTP_USER_AGENT'] variable, or with the XML if.. (I think something like <