Extension elevate Firefox from the excellent to the really, really, really awesome. They're easy to install, and can be designed to do just about anything you're heart could possibly desire. From better control over tabs to help with web design, there's something for everyone.
I'm linking to the various homepages (if they exist), because they generally have a better description of the extensions than the general sites. Firefox will automatically black them from installing extensions, remember to add them to your whitelist.
Foxy tunes - control your media player from within Firefox. It support pretty much all of the common ones. Heck, it even supports Foobar2000. Yay. It's now skinable as well, which is pretty nice.
Image zoom - from the context menu, zoom in and out of images. It's pretty nice.
IEview - opens the page you're viewing in IE. Mostly for those obnoxious non-compliant sites.
Stuff I personally adore:
Deepest Sender - LJ client. It's fully functional, and quite excellent. The music detection is a bitch to set up and the spell check doesn't work, but hopefully those things will be fixed in the future. Other than that, it works for multiple servers and it is much easier to work from the sidebar than from a separate window.
Session Saver - Reloads the last page(s) and their histories you visited every time you start up Firefox. It also works as crash recovery. I worship this extension so much. It's quite excellent, and I don't think I would be able to survive without it.
Gmail Notifier - Alerts you to new messages in your Gmail inbox. A click and you're in your inbox. It's very useful for those with Gmail out there.
Tabbrowser preferences - Tabbrowsing is god. Despite this, too many sites automaticall open new windows when we click on links, and that makes me sad. Really sad. Luckily, this extension forces those bastards into their own tabs instead. And that makes me happy. Really happy.
ChromeEdit - There are craploads of preferences that are not readily accessible. For some of them, you need to mess with your user.js or userChrome.css files. This extension removes most of the pain from the process. (Not compatable with 1.5)
Spellbound - Basically a port of Mozilla's spellchecker to Firefox. Highlight text and use the context menu to spell check. This works really nicely with comments.
Web Developer toolbar - Excellent tool. Incredibly geeky extension that has almost everything a good web developer needs. Favorite feature? Edit CSS of a page while viewing it.
So, yeah, those are some of the best. There are tons more out there, so you should go dig around the general sites yourself.
Update.mozilla.org - the "official" site. Pretty complete, but hit pretty badly when new releases come out, and is generally somewhat behind when it comes to selection and being up to date. The navigation is pretty bad as well.
Extensions Mirror - Unofficial, but up to date. It's a bit on the slow side, but it's not too bad.
Extension Room -Unofficial. Extensions nicely organized by functionalit, though that may be a turn off for some people.
Themes make Firefox pretty. Really pretty.
Phoenity - I am so in love with this theme. I go into random spasms when I update Firefox and it's not available. It's all about the clean, bright colors, and incredibly nifty subskins.
Noia 2.0 (extreme) - gray/metallic feel to it all, with some shiny icons.
Doodle - Nifty, hand drawn looking icons.
Qute - Used to be the default theme before winstripe was developed. It's still an excellent theme.
Caramel - Still not quite up to date with this release yet, but it's a gorgeous brown theme.
Aaron's themes - OK, so there isn't just one. His themes are all so lovely and pretty that I have to rec them all. Plus, he releases themes for the suite, which is reason enough to love him.
Luna - for those of you who like IE's icons better anyway.
Update.mozilla.org - most of the same drawbacks as the extension portion, but probably the most comprehensive list out there.
MozillaZine's Theme forums - Hang out with the themers, and get the latest, most up to date versions.
The nifty stuff:
The little things that make Firefox really cool.
Smart Search - OK, so maybe the title isn't very descriptive, but it's really nifty. I won't go into explaining it, as there's an example there. It works with pretty much any site with a search. Here's a list of other sites to bookmark.
More search engines - You know that nifty integrated Google search? Well you can add craploads more search engines to it.
Customizable toollbars - Right click on the navigation bar(over the reload button is fine) and select "Customize". Now you can add new buttons, remove the defaults. You can even chuck the navigation bar if that's what you feel like. If you run out of room, it's easy to create a new toobar as well.
Find toolbar/Find as you type. - no more of that obnoxious find dialogue. When you press Ctrl+F, a toolbar should appear on the bottom of the screen Type things in, and click on "Find Next". In Tools > Options > Advanced, you can also select the option of searching for text as you type it in, which can be time saving.
Setting multiple pages as a bookmark - Open up the pages you want in tabs. Go to Tools > Options > General. Click on "Use current pages". You can also open all of the bookmarks in a folder into tabs by going through Bookmarks > the folder > Open in Tabs.
Book of Mozilla - Type "about:mozilla" into your location bar.
Firefox Help - Yet more niftiness. A lot more interesting things you can do to Firefox, as well as advanced stuff. It's quite excellent.
MozillaZine Knowledge Base - A wiki of info to help out Firefox users. Still a little small, but will get better over time.
My post about RSS feeds - Goes into detail about the various RSS possibilities in Firefox/Thunderbird/Mozilla.
So, yeah, that's what I have. If there's something you were curious about, but didn't see here, feel free to ask. :) And also, feel free to pimp this post out, as I want to spread the love.