Here's my 2 cents worth, and I know A LOT of people will disagree...
Firefox is going to be just a passing fad...
It's a nice browser and all but it just doesn't work with some key apps.
( I have a helpdesk application which features a web interface for clients to submit support tickets. Won't work with Firefox. Nor windows update. Right now, it's very popular because it is fresh and new. Remember, with that rising popularity comes a price to pay.. there have already been a few vulnerabilities exposed. Every day it draws more attention to attackers and before long I think we will see it being attacked more and more.
I personaly use both IE and Firefox.