One of my favorite extensions for FireFox is FireTitle.

FireTitle was written by Jon Nowitz, a Google colleague, and it lets you name
your FireFox windows in very flexible ways.  If you do a lot of tab
browsing, it’s nice to be able to keep a consistent title for one window
regardless of which tab it is currently displaying (e.g. "weblogs", "email"). 
Press Ctrl-; and you can name the current window (which will be preserved for
future FireFox invocations).  The version on mozilla.org is still for
pre-1.5 FireFox but Jon already has updated it to 1.5 (I’m running it as we
speak) and it will be uploaded there very soon.

I also just found a pretty neat extension: 

foXpose
.  Just press Ctrl-Shift-x (or click on the icon in the lower
left corner) and it will show you an "Exposé-like" view of your tabs.  I
don’t know yet if I will find that more useful than just the titles of my tabs,
but the idea is very cool.

How about you?  What are the first extensions you install one after
installing a brand new FireFox?