Oh look, a list of my favorite things to add to Firefox!
My hands down, most used (and abused) is the StumbleUpon Toolbar. Can’t concentrate on what you’re doing? Click the “Stumble!” button and let it take you on a random trip through the internet. You can submit pages, rate them with a thumbs up or down, send a link to mutual friends, give your review of a page, and a whole slew of other goodies. (Oh, and did I mention you can select which categories you can stumble through? and that you can specify whether or not you want to see video?)
Running a close second for use is NewsFox, an RSS reader that supports Live Bookmarks. I liked live bookmarks but didn’t care for the interface. Now I have a button to click and a three-pane window (bloglist on the left sidebar, post name in the top, post content in the bottom). My one quibble with this plugin is that it does NOT open in a new tab.
In the background, quietly doing its thing is Adblock Plus. I didn’t realize this baby did its job so well until I was helping a friend debug a website and had to *shudder* open IE to view it. When I saw what I had been missing, I was truly amazed, thrilled, and well…downright speechless. (I was on her computer, since I use IE tab on my own. See below 
)
The Download Statusbar is another “in the background, doing its thing” addon that I love. It takes the traditional download progress gas gauge and sticks it in the bottom left of your browser window and out of your way.
Gmail Manager. ‘Nuff said?
FireFTP is for those “I need to upload this rightquick” moments. (I use Filezilla for site backups and other gigantic uploads).
Repagination has truly changed the way I read the web. Right-click on a “next” button, select “repagination” and then “all” and enjoy the fact that you can read the entire thing without clicking another button. *note: this puts the entire page in one window, so you will see the page headers and footers repeatedly as you scroll down*
Colorful Tabs helps me keep things organized. There are times when I can have as many as 20 tabs open at once, and keeping my place is important. And let’s face it – I love the visual of the rainbow of tabs.
IE Tab shows you how your website looks in IE without actually having to bring up the program. I use this primarily when designing, since things can render differently from site to site.
I think that’s everything. Well, actually, I feel like I’m forgetting something…