I'm currently using both IE (6,7 and 8 ) and FF as part of my job, and I have to say that IE is a complete load of rubbish – even 8. Ridiculously slow, non-compliant and completely refuses to render some stuff which has slight problems with format which FF is fine with
I use to manage a web team, we produced standard consumer and B2B sites. If the website didn't work with IE it wasn't compliant, end of discussion, because 90% of JoePublic and JoeCorporateUser use IE.
Once we implemented a new website for B2B in Norway, I knew it was going live the next day, but available on the URL from 10pm. I logged on my old home PC and couldn't access the site – rang up my lead tech to find out the problem. Turns out it didn't like IE… He was up all night fixing that one, and another step was added to the test program (check all browsers, not just the ones Techie's use).