In my Firefox 3.6.6 browser, I currently have 24 add-ons. Yeah, that’s kind of a lot, but it’s fewer than I had in older versions of Firefox. Nevertheless, if I had to pick my top-10 must-have add-ons, they would be as follows, in alphabetical order:
Adblock Plus – Pages load faster and are easier to read without all the ads. Adblock Plus not only filters the ads out, but it also automatically updates its filters from one of several rulesets that you can select from when you install the add-on.
Add N Edit Cookies+ or 
Firecookie – There are many add-ons for managing cookies, but very few that enable you to edit the content of any cookie. Add N Edit Cookies was one of, if not the, first one, though it is no longer maintained. The ‘+’ version has been updated to install on the latest versions of Firefox. Alternatively, if you use Firebug, below, then Firecookie is a plugin for Firebug that adds cookie editing capability to the debugger. Even if you’re not a developer, being able to edit cookie values is very handy for various things such as making a short-lived session cookie stick around for as long as you want.
Firebug – This is the ultimate debugging environment for Firefox. Whether you’re working on HTML/CSS or JavaScript/AJAX, Firebug enables you to see and tweak things on the live page in the browser. There are too many capabilities to list here. If you don’t already know about Firebug, check it out.
Greasemonkey – Know a little JavaScript? Then you can write Greasemonkey scripts to add, remove or modify the contents of a web page when Firefox loads it. To see what’s possible, take a look at some of the scripts at Userscripts.org as well as the online book “Dive into Greasemonkey”.
Live HTTP Headers – If you need to see the actual conversation between your browser and the web server, this is the add-on for you. It’ll record and show you the header information for every file request and response as it happens. You can even modify the header information for a request and resend it.
NoScript – Letting JavaScript code from any web site execute in your browser isn’t particularly safe. Some scripts can be downright annoying. NoScript enables you to decide which web sites are OK to run scripts and which aren’t.
Session Manager – Although Firefox has a built-in session manager to restore your tabs when the browser crashes, this add-on goes much further. With Session Manager, you can save multiple sessions with their own sets of tabs and windows with their corresponding back button histories. Tame your myriad tabs by creating a session for work, one for social networks, another for your hobby, etc. In addition, you can set them to be auto-save sessions so you don’t have to remember to save the current one when you switch to another.
Tab Mix Plus – Each version of Firefox makes tab functionality better, but Tab Mix Plus gives you more control over them, including the ability to duplicate tabs, control tab focus and lock tabs. Although it has its own session manager, it is aware of the Session Manager add-on and won’t interfere.
Vacuum Places Improved – Do you find that Firefox’s address bar and form auto-complete start to get really slow after a while? It’s because the sqlite database containing all that information needs compacting and this is the add-on to do it easily. It makes a big difference.
Web Developer – Too many useful features in this add-on to list here. Some of the ones I use most include resizing the browser to specific dimensions (e.g. 1024×768), outlining specific types of HTML elements on the page and extracting the actual HTML source from the current page (including dynamically-added content that Firefox’s View Source leaves out).
Bonus add-on #11: Adblock Plus: Element Hiding Helper – Although Adblock Plus does a good job of removing ads, sometimes there are other page elements that you want to get rid of. The Element Hiding Helper works with Adblock Plus to enable you to point at a block of content on a web page with your mouse and make it go away for that specific page or anywhere on a site.
Since I use Firefox for development as well as surfing, many of these add-ons are developer-oriented. Your most-useful add-ons will depend on your usage and may be quite different than mine. That’s the beauty of Firefox’s add-on community–so many enhancements to fit your needs.
What are your favorite Firefox add-ons?