This is a list of the Firefox extensions I use. When there’s an asterisk (*) that means that I don’t keep them enabled and only turn them on when I need them.
Aardvark*
Aardvark is a great tool for inspecting a webpage’s elements. http://karmatics.com/aardvark/
Adblock Plus
If you have to ask… oh hell, it blocks advertisements, okay? http://adblockplus.org/en/
All-in-One Gestures
You can do gestures to cause actions. I don’t, I just use this to enable the scroll wheel on the tab bar. http://pagesperso-orange.fr/marc.boullet/ext/extensions-en.html
Bookstack
… Eating my own dogfood since 2007. Shove some links in, and pull them out one at a time. http://code.google.com/p/bookstack/
CS Lite (CookieSafe Lite)
Used to whitelist sites that can store cookies, both temporarily and permanently. http://forum.softwareblaze.com/
DOM Inspector
This is probably included with your Firefox install. Lets you inspect the DOM of webpages and even Firefox itself. Introspection. https://addons.mozilla.org/en-US/firefox/addon/6622
Download Statusbar
We don’t need no stinkin’ windows to tell us what files are coming down. Puts a bar just above the statusbar to display your downloads. Highly customizable. http://downloadstatusbar.mozdev.org/
Firebug
Great tool for web development. Let’s you make changes in real time. http://getfirebug.com/
Honorable mention for Fireunit (http://fireunit.org/) which I am looking forward to trying when the 3.1 betas are stable enough for me to update my extension.
Greasemonkey
Modify any and all pages and even Firefox itself via Javascript. You have access to scripts others choose to publish as well. Huzzah! http://www.greasespot.net/
NoScript
Named after the tag that denotes content to display if a browser has Javascript disabled, NoScript bestows extra security and saneness. Whitelist sites that you wish to privilege with the ability to run scripts, or temporarily allow those that you visit rarely. http://noscript.net/
Organize Status Bar
Allows you to configure the order of items appearing in the status bar. Simple and to the point. http://yellow5.us/firefox/osb/
RefControl*
Allows you to set what gets sent as the HTTP referrer to a site. Sometimes this is useful, but the sites I use regularly don’t need it so I leave it off. http://www.stardrifter.org/refcontrol/
Stylish
Like Greasemonkey, but for CSS (Cascading Stylesheets). Sometimes you don’t need a script. A beaut for hiding, resizing, positioning, coloring, and more. Also like Greasemonkey you have access to styles others choose to publish. http://userstyles.org/stylish/
TableTools*
A nice little extension that allows custom sorting and filtering of HTML tables. When I hit a statistics page that doesn’t do this in Javascript, I reach for TableTools. http://www.mingyi.org/other/tabletools.html
User Agent Switcher*
Like the RefControl extension, this is damn useful when you need it. Especially for sites that don’t know what Linux is. http://chrispederick.com/work/user-agent-switcher/
Web Developer*
Like Aardvark, DOM Inspector, and Firebug, the Web Developer extension is great for web development. It has a broader focus than the others; this is the swiss-army web development extension. http://chrispederick.com/work/web-developer/