The latest (re)incarnation of the Mozilla project:
(Now has the name Firefox)
FireBird is a nice browser, although quite basic in terms of functionality unless one installs extensions (I use TheProxomitron, so it's blocking functions are useless to me). Gesture, Tab and several other extensions are essential to me before the browser becomes really functional. Memory usage is now almost the same as Opera 7 (kudos to the developers for the accomplishment of streamlining XUL so well!), although startup time is longer. A good browser, but i still find Opera superior in many respects.
I am quite disappointed on a conceptual level however, because I think something so stripped down does not challenge a novice user to be adventurous with new features. The fact that extensions are available does NOT mean most people will ever use them (
possible ~= probable). Sadly users have become accustomed to a poor user-interface through IE, and don't realise there is so much more that can help researching and navigating the web. Opera's approach is the reverse - include rich functionality and then let users remove what they don't need (see RemovingMail for an example). A friend who used mouse gestures in Opera LOVED them, but if my friend had used Firebird, she wouldn't have ever known what they were in the first place. Opera provides a streamlined and highly integrated core set of useful functions, that new users get to use from the beginning - they can be easily turned off if not needed.
The fact that the Opera executable is 2X smaller than FireBird when it packs the functionality of a great many different extensions highlights the relative nature of FireBird's "lean'n'mean" credentials (unless you are refering to Opera's 'bloat' on the metaphysical plane? :). Don't get me wrong, Gecko is really great, and FireBird is vastly more accomplished than IE; but it is going in the wrong direction in trying the lowest common denominator in my opinion. It seems to be slowly turning Mozilla into merely an extensible IE clone...
The very BEST thing about Firebird for me is the transparency of the program due to it being open-source. Every bug is fully viewable and anyone can contribute to their resolution. I don't think open-source is 'always' better than closed source, and I actually think that Netscape suffered the fatal blow in it's competition against IE in part because Netscape 5/6 took such a long time to develop (and was so bad when it finally emerged 2.5 years later, the 'from scratch' Opera 7 took ~1.5 years from start to finish), lots of devoted Netscape users jumped ship to IE5/6 during Netscape's bold open-source experiment. Neverthless, it now means there is a product (with small market share), that allows any user to contribute to it.
 The mouse gestures extension are currently clearly inferior to Opera functionality (see WhyOpera), but the tab browser extensions are brilliant!, having lots of options that Opera lacks. The tab preference settings UI is very dense and somewhat confusing, but packs a serious tweaking punch. I find TTLO extensions essential to regain client-side MIME editing (and easy access to HTTP/1.1 settings), and Preferential for better config editing (although it is quite buggy). I use the niceQuick Prefs V1.0 to replace the excellent F12 in Opera. QuickNotes I've installed, although still
very basic compared to Opera's Notes panel. And paste and go V0.2 which mimics another Opera feature works, but I can't get it to bind the key that I want. I've also got Image zoomer V0.1 to compensate for Firebirds lack of this ability, although it is currently still quite clunky (only half/double size). I've installed Contextmenu thinking it would do an Opera "User CSS" thang, but it cannot toggle style-sheets on/off. Lots of stuff in there though.
 As has been pointed out, Opera compress their executable and FireBird don't (Opera.exe is therefore 5X smaller in the default installed configuration), the 2X value comes from comparing compressed EXEs. Firebird still has 6X greater additional DLL/EXEs than Opera. I don't want to descend into mere number counting, but because many users consider Firebird to be less 'bloated' than Opera, it is at least worth highlighting. By the logic of 'bloat', Firebird is much more bloated than IE and Opera because it contains features IE and Opera do not? :)