I'm a web developer / programmer, and I use Firefox as my main browser (due to Firebug and FireFTP, mostly). I then test in IE9, IE8, and whatever the latest version of Chrome is afterward.
For very thorough testing, I throw in IE7, Safari, and occasionally others. We used to take care to always include any browser / version that had a large following, but we've recently stopped supporting old / obsolete browsers. If you're still using IE6, Firefox 1/2, etc., sorry, you're SOL. It might seem a bit callous or perhaps even stupid, but it's the only way we're going to make progress. Coding websites for IE6 is a nightmare. Even IE7/8 are still a PITA.