I used to only use I.E but I got tired of the slowness, the issues which comes with using it and the fact that it was so easy to become a victim to those annoying hijackers scripts. I've never had much issues with firefox, I think the worst that I have experienced is strange crashes after upgrading but this hasn't happened very often and I like being able to install plugins and spark the appearance up. I would never drop firefox for another browser, I don't see how any other could increase the speed worthy enough to switch or offer better protection.
Well Google Chrome to me is faster and has extensions, and themes. Also, when they did a contest too see which browser they can hack the fastest Chrome was one of the last ones they were able to hack.
Currently using Chrome 9 beta, I used Firefox beta 8 for a while but didn't really like it. One problem I'm having with Chrome is if you stay on a webpage to long it wont reload the page, and another problem is it wont load Youtube most of the time.