I've been using Mozilla Firebird for a few months now, and have loved using it since my switch from IE 6.0. The fact that there is tabbed browsing, pop-up blockers, and a great support for web standards are some of the reasons I made the switch. I only recently discovered the great extensions for Firebird that can make the browser that much more useful.
I recently installed the CSS extension, which allows me to edit sites style sheets on the fly and test out things as I'm browsing. I'm sure I'm just telling you something you already know, but give it a whirl. It's a great utility, and is seamless in operation.
Update: Also installed StedSidebar, StatusBar Clock, CuteMenus, and TabBrowser Preferences.