I don't like running IE, but there are a few sites that still require it to work properly. Fortunately, with this add-in IE is never more than a click away when you need it.
Focus Last Selected Tab
As far as I'm concerned, FLST makes Firefox behave the way it always should have. With this extension, when you close a tab, the previously active tab gets focus. Particularly useful when searching the web and launching new tabs when exploring search results (something I do a lot).
Because some times those images are just too darn small. Great for zooming into screenshots.
I hate navigating to a site and being blasted by a flash animation or ad (especially when they have sound). This extension lets me control when those animations play. It desperately needs a whitelist feature, though - especially since more companies are incorporating flash into regular old business web sites. UPDATE - the latest version does include whitelists. Awesome.
Because Google is the source of all knowledge.
Opens the currently viewed URL in a new tab. Not something that I use constantly, but every once in a while it comes in handy. The "Merge Windows" feature is particularly handy - it combines all the open Firefox windows into one.
Uh - it blocks ads.
Disable Targets for Downloads
Prevents annoying blank tabs from popping up when downloading files from some sites.
I learned about this one from the PCMag article. Allows you to save a web page or snippet of a page into a library for future viewing and searching.
Web Development Extensions
An option in the advanced Firefox install. It gives you a nice tree view of the current DOM document. Very handy.
User Agent Switcher
OK, I haven't actually needed this yet, but it's there just in case. :) Allows you to switch the browser that is reported to the server when requesting pages.
A fantastic grab-bag of developer-related functions. From disabling cookies and scripts to viewing style and tag information, this is a must have extension for web developers.
Live HTTP Headers
View HTTP headers on your web requests - great for troubleshooting problems.
Another one that I learned about from the PC Mag article. A handy little tool that lets you view the RGB color definition for any point on a page.
Provides easy access to the various user customization files of Firefox (userChrome.css, user.js, etc)
A fantastic tool for keeping tabs on your Gmail account.
How about you? What extensions do you run?