my main reason for firefox is the ad-block plus plugin. Chrome nor IE have anything near as good.
If Chrome ever had an adblock that worked that well, I'd give it a try.
I'd also like a 64 bit firefox which would be pointless until we got a 64 bit flash.
I also hate how if your doing a clean windows install and you backup your bookmarks in IE, it doesn't keep the order.. its damned annoying. Another win for firefox (and probably chrome).