If you haven't recently, then check out the 101 reasons to use Mozilla Firebird. What extensions do you use? What extensions would you like to see? I'm currently using TextZoom, RSS Reader Panel, Mozilla Google Toolbar, Mozilla Tinderstatus, Linky and LinkVisitor. TextZoom makes surfing on a 1600x1200 laptop screen much nicer. RSS Reader Panel is a simple RSS feed reader for Firebird's sidebar. I don't actually use the Google Toolbar (I've got it hidden) but the extension provides a nice "Translate to English" context menu item that I use a lot. Tinderstatus lets me know the state of the Mozilla tree. I use Linky for lots of my astrophotography (and other) image viewing and LinkVisitor makes it much easier to recognize the new articles at sites like The Inquirer and The Register.
update:I just noticed that Disruptive Technology posted basically the same topic earlier today, even asking readers which extensions they use. It's a nice blog so go give it a look.