Since a little bit before the 1.0 release, I've been pretty satisfied with Firefox. I use NetCaptor (an IE shell with tabs and other functionality) and IE at home, and Firefox only for testing web pages. At work, I use IE for intranet sites and a few selected websites, but Firefox for just about everything else. I'm happy to say that after finding the extension Tab Mix Plus, Firefox' awful tab support has been transformed into something perfectly acceptable and good enough for my everyday use. It now seems pretty likely that I'll move to Firefox as my primary browser at some point just so I use the same thing at home as I do at work, and I can't use NetCaptor at work because of the license.
As more and more sites redesign to not suck on Firefox, using it has become much more pleasant. I still think that people who insist that Firefox is faster and more stable and more secure than IE are deluding themselves—I have far more crashes in Firefox than I do in IE (Firefox and NetCaptor crash occasionally; IE never crashes for me), and I could go on about the number of security patches released for Firefox versus IE, but I won't. IE7 is still has the possibility to win back my heart, but not if its tabs continue to suck just as much as Firefox' without extensions.
It's honestly been a while since there was even any competition for the web browser to win my heart—since the first beta of IE4, to be exact. Sheesh, that's been like 7 years. It's really nice to suddenly have two browsers that I don't mind using again.
I've been using Firefox to some degree since it was Firebird (though I don't think I really used it way back when it was Phoenix), but it wasn't until a month or two ago when I discovered Tab Mix Plus that I really felt like it was a worthy competitor to IE. I still don't like having to install an extension to make its star feature usable, but I can deal with one add-on.