I use Firefox mostly, sometimes with Tor, and occasionally Chrome and even Opera too. I use an older version of IE to see if pages look correct on it,
Browsershots.org is useful for this kind of stuff.
and also when I want to grab something from the cache but don't want to sift through my entire cache folder on Firefox.
There are programs out there which are helpful for this. Ever since Firefox started breaking up and distributing the cache files into hundreds of folders, I've been using
VideoCacheView.
I also knew a computer science professor who preferred to use IE. That was a bit unexpected. Then again, I haven't used the latest versions of IE either and I've heard that the newer releases have improved quite a lot since the XP days.
One thing that has improved quite a bit since then is standards compliance.