My main browser is Firefox on XP, but i test in all the ones i have installed here .. Firefox, Mozilla, IE and Netscape (all latest versions). I can switch to SuSE Linux and test Konqueror or Galleon. For the older versions (and for different OS & resolutions) I use the excellent
http://www.browsercam.com/ 
I know what you mean about 95% (or whatever) of people using IE. But it depends what kind of site you're developing in some ways .. a site relating to design is quite possibly going to get a lot more mac visits (perhaps in Safari) and so on.