Windows only: The Firefox Extension Backup Extension (FEBE) lets you backup all of your extensions, themes, bookmarks, preferences, and even cookies (you can pick and choose if you don't want to backup everything), while it's counterpart, CLEO (Compact Library Extension Organizer) bundles everything into a single .xpi file that you can install on any installation of Firefox.
FEBE and CLEO together do the job of my recent Firefox extension packs feature - but the whole process seems a bit easier. The extensions are still a little buggy (I had trouble getting CLEO to work), but this is a nice step forward in making extension packages that are easy to move around and install. You can package up all of your extensions, save them some place handy (I'd probably keep one in my Gmail), and install the .xpi any time you've got a fresh profile or install of Firefox.
Windows only, for now.