Theme: RedShift v3 - I like black/red color schemes (as you might be able to tell), and this one is awesome.
4chan - Adds various controls to the boards and threads on 4chan such as quick reply, thread collapsing, and a thread watcher.
AdBlock Plus - I kinda liked the original better, but it seems to have died off while the inferior AdBlock Plus lives on. I had a custom filter list that totally annihilated ads without false positives in the original. Constructed by manually blocking each one as I came across it. A slight difference between the original and Plus is that the original could be configured to prevent the browser from going to sites in the list, even if they were entered in the location bar. That feature is strangely absent from AdBlock Plus. Which leads me to the next extension in the list...
BlockSite - Expressly created to provide the site blocking functionality that the original AdBlock had that's gone in AdBlock Plus. I use it to block DownloadHelper's page it loads after an extension update, and mIRC's nag-you-to-register web pages (both of them).
BugMeNot - Ever wanted to read an article on nytimes.com or washingtonpost.com and gotten that annoying "you must register a free account to view our content" bullshit? BugMeNot.com exists to counteract that. It's a community-collaborative effort. People register dummy accounts using fake information and submit those accounts to BugMeNot, which you can then use by going to their site. This extension just streamlines the process, enabling you to log in by right clicking and saying "Log in with BugMeNot". Also works wonders with IMDb.
DOM Inspector - Allows you to view the document object model of a web page, or even that of a Firefox window itself. Pretty handy for a web designer. I use it a lot in making styles for Stylish.
DownloadHelper - Allows downloading videos from flash video sites like YouTube.
Download Manager Tweak - Customizes the download manager, allowing you to open it in a window, tab, or in the sidebar. All the other download management extensions are crap compared to this. I'm looking at you, Download Statusbar. (I used Download Statusbar for a while during the tense period between updating to Firefox 2 and waiting for Download Manager Tweak to be updated for it)
Fast Dial - Visual bookmarking. Whenever you open up a new tab, it shows bookmarks you've configured it to show, complete with either a thumbnail of the website or a logo you've told it to use for that bookmark.
FireFTP - Firefox does have a built-in FTP client, but it only lets you download. This is a full-featured graphical FTP client that you can open in a tab or a separate window. It will store authentication information so all you have to do is select the server you want to connect to and hit Connect.
GMail Notifier - Tells you when your GMail accounts have new email. Supports multiple accounts. Clicking it will open up GMail and let you get right to reading your new mail.
Not4chan Extension - Created for use on not4chan, but works on 4chan and a couple other *chans. Makes mass-downloading the images in a thread easy. The site says it hasn't been tested in Firefox 3, but I can verify that it works. I'm actually using an old version (1.13) that I hacked to change the maximum version of Firefox it would install on. XPI files are just ZIP files renamed. Whee.
Screengrab! - Takes screenshots of web pages. It will do just the visible portion, the entire page, or an area you select. It'll save it to your hard drive or copy it to the clipboard so you can paste it into photoshop or whatever.
Smart Stop/Reload - Merges the Stop and Reload buttons into a single button. It makes sense, since you only ever really need to do one or the other at any given time.
STOP! Hammertime! - Whenever you hit Stop (or its keyboard shortcut, Escape), this plays the sound clip its name comes from. Lawyers are trying to make this extension disappear. Isn't it sad that they've got their panties in a bunch over a two second long sound clip?
Stylish - Modifies the look of websites by allowing you to specify custom CSS to use when that site is loaded. Can also modify elements of Firefox itself, since that also is defined via CSS. I've used it for a number of reasons, both on websites and on Firefox itself.
Tab Mix Plus - I thought that this might have been obsoleted by Firefox including a session manager as a standard feature beginning with Firefox 2, but it offers so much more that it's still good to keep around. Of all its features I primarily use the Closed Tabs List toolbar button and the tab bar New Tab button. I also have it set up to hide the tab bar close button and the all tabs list dropdown (which is accessible via Ctrl + Tab) and only ever show a close button on a tab when that tab is being moused over.
Now you know.