What do you use?

I'm using Ff 3 beta 4 so some of mine are not working yet, here is the list anyway
Adblock Plus- works
ConQuery - still broke
CookieSafe - still broke
DownThemAll - works
StumbleUpon - works

I got a little timer that counts the days till regime change.

Nice!

Well I've had a request to explain what these addons do.

Adblock Plus
Uses a filter set to block online advertising. The filters I use are updated online and automatically updated, so there is really little to no interaction required on my part to keep it useable. There are other things possible with this, for instance blocking text ads , I just do not care enough to do it. The basic operation is quite fine.
ConQuery
This lil guy allows me to highlight a word and right click it and search, in a new tab, with whatever search I'd like to use that I have installed via the search bar. Unfortunately this one is not updated yet for use with Ff 3.
CookieSafe
As with the last one this is not updated either. This one gives me access to complete cookie control from the status bar. I can do any number of things from viewing cookies, to deleting them or allowing them temporarily on an as needed basis.
DownThemAll
These last two are working in Ff3. DownTehMall as I like to call it, works by allowing me to download all the links on the current page. From videos and music to basic text files. This list you will create can then be downloaded all at once or queued. Careful though, this will rape any number of sites. So if you know the person/site or just don't care, plan accordingly.
StumbleUpon
This one is good for those rainy Sundays or hungover Saturdays. It is a random link generator. Those it isn't so random. Its based off your like and dislikes. Pretty cool. There is also a social aspect to this but, I just use it for the links.

These are my favorites-

Foxmarks Bookmark Synchronizer - its in beta testing now that you have to sign up for but works perfectly for me

ColorfulTabs - an excellent visual addition (you can get the version for FF3 here - http://www.thestasis.com/?q=colorfultabs)

Auto context - Auto Context is an extension that will open a customized popup menu when you select a text. (works with 2.0 – 3.0b3 though it will work with FF3b4 if you edit the xpi file)

Down Them All - an excellent download manager/accelerator

Noscript - It allows JavaScript, Java and other executable content to run only from trusted domains of your choice, e.g. your home-banking web site, and guards the "trust boundaries" against cross-site scripting attacks (XSS).

And I also use Adblock Plus

Syd, here is Cookie Safe Lite (updated for at least FF3 beta4)
CS Lite
Very nice extension that allows you to control cookie permissions.

I use the Mythical Sirens Summer Night theme. Maybe not as useful as the other add-ons, but transforms Firefox into something really organic and nice

Syd, can't really help you but, looks like the others have pretty much covered that. Just wanted to suggest that if you DO NOT have your browser set to upgrade, do so. I know the beta versions are really buggy. I just upgraded mine to RC2, but actually it came with my second Linux Mint 5(048) beta which seems to be working well with this browser version, the other FF beta version they had really had some problems.Your browser really effects the way your system performs. just my 2 cents, even though off topic.

I'm using Ff 3 RC1 and I'm down to 3 add-ons. I don't like nor need much more than that anymore. I see RC2 just came out. I'll try it out, hopefully my add-ons are not broken. Nice to meet you golden.

Here's another addon that I use that might help you out Syd-

MR Tech
"The primary goal of this extension is to provide the tools needed to install and manage extensions and themes locally.
To do this the extension provides multi-extension installation support, hacking capabilities to the Extension/Theme manager windows, features to find and troubleshoot Extensions/Themes Build, GUID and Profile information."

The main reason I use it: You can turn off/on extension compatibility checking very easily. Meaning, you can force your older extensions to work with FF3.

I'm actually down to 1 add on, it feels nice now that Ff3 is out. Too much functionality defeats the purpose.

web developer is my fave.

there is a new one that restores form info to page info, which was left out of firefox 3 for some reason. i don't know the name, i just know that it is indispensable.

boosh.

One of my favorites is "customize google". It allows you to control your interaction with google and what info they can collect from you. Google collects a *LOT* of information from most every site you visit and much of that info can be linked back to you.