I use a couple of mozilla right now:

• The one, I really enjoy is Mozex, which enable the use of external programs like mutt or, emacs for mail.. text editing and a lot of other.
• For CSS/HTML debugging i use another one which is really kool too.

But i guess the ultimate is QuickManager. This beautifull extension allow the user to browse the mozilla extension database, and install extension with a single click. (same can be used for themes)

So now, the first thing to do install QuickManager :)