I used to use IE, then switched to Firefox when they introduced tabbed browsing although I found it too slow and clunky, so when IE added tabbed browsing, I switched back.
From a developing perspective, FF is more compliant but that means more strict and IE is more forgiving. The sensible option is to develop in FF because it’s more likely to work in IE by default, but to be honest FF drives me nuts so I only worry about IE and if it works in FF aswell, it’s a bonus. As an aside, 17.1% of all my users use FF.
I must admit, I don’t really understand why people prefer FF myself. And life as a developer was so much easier after Netscape bombed and we only had to worry about one browser 