I have more issues with FF than IE. I still run XPpro on my PCs, even my new PCs recently. Using IE6 still. But really need to get v7 working. It's more secure. Tried upgrading to IE7 and it goofs everything up. FF has become a resource hog once you upgrade to version 3. So I went back to v2.
In general, I do like FF but I've rarely had issues over the years with IE or Windows for that matter.
IE still dominates so people still need to make their sites useable in IE. For example ... I see lots and lots of blogs that are messed up in IE. It's obvious that most aren't checking their blogs in IE also. Theme designers too.
My 3 centz anyway ... heh heh