Slashdot is reporting that Mozilla Gains on Internet Explorer. I can say personally that I have switched entirely from IE to Firefox (0.9.1). Unlike the earlier version I tried (then named Phoenix), Firefox has been very fast, very stable and very ... fun. Just love the tabs.
I keep IE around for occasional compatibility testing, and for the couple of sites I use that don't like Mozilla, such as Shutterfly.
The only thing I miss is InstantSource, but I have contacted the authors about porting it over, and perhaps they will. I'm sure I'm not the only one. The WebDeveloper extension for Mozilla is really good, and has some great features that InstantSource doesn't have, but is not as useful as InstantSource.