I tried Mozilla Firebird as soon as it was announced, and was not impressed enough to switch from Mozilla. Plus, I was using Mozilla Mail as my email client, and it felt a bit funny to use something else as a browser.
Lately I started using Evolution, which I like a lot apart from some minor annoyances (no context menu in the message pane for copying, poor LDAP support compared to Mozilla Mail), so I went around shopping for a new browser.
I tried Epiphany, which is still a bit rough, and Galeon. One or the other of them messed up my Mozilla installation (on two different machines, so it's not a localized issue) so much that I had to wipe it and reinstall it from scratch.
This morning while having breakfast an entry on AMK's Diary mentioned Burning Edge, a Mozilla/Firebird blog where new builds are announced daily. I went to have a look and there's a Linux+gtk2+xft nightly build. I installed it and liked it so much it's now my default browser. Definitely recommended, if you can overcome the .rpm .apt addiction and simply untar an archive somewhere. Of course, using Slackware I'm not annoyed by these things. =)
update: mainly as a bookmark for myself, Firebird packages for Slackware 9.0 are available at pryan.org