There are so many good ones. Here are just two of the top of my head (I'm sure there are other better ones)...
Yoono - Combines almost every social communication application into one handy (collapsable) sidebar in Firefox. Everything from Twitter and Facebook to AIM and Yahoo! Instant Messenger (there are others, too) are supported.
It has pretty good customizability, some surprising functionality, and is getting new updates and tweaks very frequently.
If you're the type of person who likes to stay in touch with everyone instantly - and non-invasively - while surfing the web, then this add-on might just be for you.
Greasemonkey - possibly the most powerful add-on out there, this thing is customization to the max.
GM allows you to install/alter/restrict/create/etc. pieces of java script to fundamentally alter (ie. improve) the way websites show content.
It's a tad complicated for newbies, but can be well worth the research.