Here’s my list of FireFox extensions:

Web Developer - excellent toolbar for use if you’re a web developer, includes outlining tables, block level elements, css viewing/editing inline, header information, much much more (needs FireFox installed with Developer Tools).

Adblock - One of the best extensions I’ve seen. Lets you weed out ads, using filters with wildcards, for example never load any iframes or images from “*/ads/*” or “*doubleclick*”. You can import my list here.

QuickTabPrefToggle - This one will intercept popups and new windows and open them as tabs insteads (customizable).

WebmailCompose - This intercepts “mailto:” links in web pages, and opens your favorite web-based email, instead of Outlook or whatever the default mail client is on your operating system.

PRGoogleBar - Includes Google PageRank value in the open source clone of the original google toolbar.

Autofill - Save form information for auto filling.

user Agent Switcher - another handy web developer tool, to simulate what would happen with a different user agent.

BugMeNot - Great for getting around those pesky registration forms on sites like the “New York Times”.

Bookmarks Synchronizer - Good if you have more than one computer, and want to sync your booksmarks (requires ftp or scp server).

TinyURL - Great for posting in forums, or in email when sending long urls. It will shorten to something like http://tinyurl.com/a24aa

Netcraft Toolbar - This is handy for reporting phishing scams.