Celebrate Freedom With Firefox Extensions
Gmail Manager
For folks who use Gmail, Gmail Manager is a must-have. It lets you log into multiple accounts, notifies you when you receive new messages, and... well, really that's about it. But it does those things very well, and makes Gmail a viable alternative to a desktop mail client.
Greasemonkey
Greasemonkey deserves more space than I can give it here, because the potential uses for the app are infinite. You see, Greasemonkey lets you integrate your own code into other websites. Want to add a list of keyboard shortcuts to your favorite website's post form? Piece of cake. Want to add popup displays to every image on your site? No problemo! Want to run your own code and add information from a third site? For sure! There are dozens of scripts available that add functionality to sites like Flickr and Gmail as well.
IE Tab
As Gordon has colorfully discussed on the No BS Podcast, we use Outlook for email at Maximum PC. Unfortunately, Outlook's web interface doesn't work particularly well with Firefox, so I've installed IE Tab solely for the purpose of checking work email at home.
Mouse Gestures

It has been many months since I last clicked the back button in Firefox. Instead, I right-click my mouse, and drag the cursor to the left. Instead of moving my cursor from the page every time I need to hit a nav button, I use a simple mouse gesture. Up, down, up reloads the page, down and right closes the tab, etc. Plus, the plugin makes cool red lines across the Firefox window.
Operator
Operator is one of the first pieces of software that can read microformats--small chunks of meta-information embedded in web pages that's designed to be easily readable and understood by other software. Photos on Flickr can have microformats that embed the latitude and longitude they were shot at. Contacts on Twitter have their user information embedded in a different type of microformat. Odds are that microformats will be fully supported in Firefox 3, but until then, you can test the waters with Operator.
Pennypacker
Unlike my pal, the Vede, I like Penny-Arcade and think those guys are really funny. But, I don't think that their site navigation and poor search engine are a laughing matter. Especially when I'm looking for a particular comic, whether it's about the original Xbox controller, this punny gem, or even something patently offensive. Luckily, the PennyPacker extension (also available in handy Greasemonkey format) gives you better search and tagging for every Penny-Arcade strip ever released, so you can hop skip and jump to your favorites.