Here's your cheat sheet on the absolute best plugins for Firefox -- that you probably don't know about.
1. Autopager. You know those annoying " 1 2 3 4 5 " pagination buttons at the end of each page of results? This awesome plugin does away with them, making websites one long scrolling result. Niiiice. (Yes, even APC has the annoying pagination buttons… a necessary evil to reduce server load, unfortunately, but er, we can't stop you using Autopager with APC.)
2. Add to search bar. Add any site's search box to your Firefox search engines list, alongside Google -- even the APC search box, for example. All you have to do is right-click in the search box on any site and select "add to search bar".
3. Foxmarks. The essential bookmark sync -- it syncs your bookmarks to a cloud server, or one of your choice, and can keep other copies of Firefox on other computers in sync. Safari for Mac and Internet Explorer versions have also been released, so you can keep those browsers in sync too. It can also sync website passwords, but make sure you use a "master password" on your browser which forces anyone using your computer to type in a password before any other passwords will be autofilled. Foxmarks makes setting up a new computer a piece of cake, because your bookmarks will just 'be there'.
4. PearlCrescent Pagesaver Basic. Saves an entire web page as one long screenshot (even including Flash content!). Without doubt the easiest way to save a page just how you see it in your browser in order to email it to someone. Every other way (saving as a PDF, emailing the HTML) inevitably results in a mangled page at the other end.
5. SearchIMDB. You know when you're hunting for a movie and you get the movie listing up, but there's no reviews of the movie there? You have to highlight and copy the movie name, open a new tab, go to IMDB, paste the name of the movie, press enter… and all that just to see a star rating and a few reviews for the movie! This handy extension allows you to highlight the movie name (or any text on a web page) and in one click get an IMDB search result up. So useful.
And… plugins that offer small improvements
Fission. Put the page loading progress bar in the browser address bar, where it should really belong. Why waste space on it elsewhere, when you're already looking at the address bar when you type an address?
Google Gears. Allows websites to store data on your computer (with your permission). For example, it allows you to cache your Gmail locally so you can log on and use Gmail even when you have no net connection -- such as on a plane. It also allows Google Reader to cache RSS feeds locally so you can read them while offline and still have your read items status synced back to Google when you go online. Other websites use it to speed up page loading because they can cache a lot of their site code and images locally -- much more comprehensively than the standard browser cache can.
Stop or reload button. There's really no need to have a separate stop and reload button -- you can only ever be doing one or the other at a time. So this extension merges them into one, with the correct symbol shown on the button according to what you're doing at the time -- currently loading a page, or looking at a fully loaded one.
Plus… essential plugins for Mac users
Skip this if you're not a Mac user, but since I am, I thought I might as well add it for the reference of other Mac users.
1Password - not exactly a plugin for Firefox, though it does come with a plugin for Firefox. It's a password manager that generates secure passwords and remembers all your passwords across all sites. The best thing is it works in all web browsers for Mac, allowing pain-free autofill of passwords. Plus, it's highly secure -- you can set it to auto-lock your keychain when you sleep your Mac, or even a number of minutes after you unlock it, to protect from snoops or against theft of your notebook. Way more secure and convenient than any in-browser autofill. (You can sync your passwords between Macs too.)
Firefox PDF Plugin for OS X: Displays PDF documents in the browser, just like in Safari. Why this isn't the default behaviour of Firefox is a bit of a mystery.