Here is a list of firefox add-ins I use every day:

The most important is LOL:

https://addons.mozilla.org/en-US/firefox/addon/1341

This extension has a really bad name, but it’s very good. What it does is when you hit space it puts a number on each visible link on the page – if you hit that number the link is pressed – MouseFree computing here I come!

It is the predecessor to Hit-a-hint, and when googling for lol I always enter hit a hint in order to find it.

Google toolbar for Firefox

http://www.google.com/tools/firefox/toolbar/FT5/intl/da/index.html

Goes without saying, if you use gmail and other google tools, this is an essential.

Ubiquity

http://labs.mozilla.com/2008/08/introducing-ubiquity/

This is a nice concept, that I am trying to implement in my every day use of firefox, it is a tool for doing normal day tasks, like finding a map, or sending an email, with very few keystrokes – go check it out.

Personal Menu

https://addons.mozilla.org/en-US/firefox/addon/3895

Removes the menu bar and gives you three buttons next to the search box, gives you more screen space.

Twitterfox (OK, I can actually live without this one, but it’s good)

http://twitterfox.net/

Twitter embedded into firefox, works very well…

I must say that I do prefer tweetdeck, but twitterfox is good.

Edit:

Rasmus Kromann-Larsen pointed out Download statusbar to me.

https://addons.mozilla.org/da/firefox/addon/26

I don’t have much experience with it, but it seems neat. What it does is it lets you get rid of the download dialog, which in my opinion, is always in the way.