After reading Joel Spolsky's and Jon Udell's posts on Mozilla Firebird I installed the browser this morning. After using on and off during the day it works great. The tabs alone make this much preferable to IE. It feels fast too. As a few people have commented, if Microsoft are not releasing the IE replacement for another couple of years (at least, based on previous OS release schedules) there is a lot of scope for competitors to grab market share.
High on my wishlist would be a decent managed API to embed the Mozilla browser within a WinFoms application. Those IE COM APIs are hideous to work with.