Add-ons

FlashGot Mass Downloader by Giorgio Maone

Download all the links, movies and audio clips of a page at the maximum speed with a single click, using the most popular, lightweight and reliable external download managers.

User Agent Switcher by chrispederick

The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.

Element Hiding Helper for Adblock Plus by Felix Dahlke

Fight the text ads! Element Hiding Helper is a companion extension for Adblock Plus meant to make creating element hiding rules easier.

Live HTTP Headers by Daniel Savard, Nikolas Co

View HTTP headers of a page and while browsing.

HttpFox by Martin Theimer

An HTTP analyzer addon for Firefox

Mozilla Archive Format, with MHT and Faithful Save by Christopher Ottley, Paolo Amadini

— Open web pages saved with Internet Explorer or other browsers (MHT)
— Save many tabs, video and audio reliably, in a single ZIP file (MAFF)
— Return to the original site you saved a page from, and more!

Telify by michael-koch4096589

Converts telephone numbers into clickable links for use with CTI applications, SIP clients, Skype, Netmeeting, snom phones, the AGFEO TK-Suite Client and SerVonic IXI-PCS.

Collector's Note I prefer this over the default Skype addon as it allows me to configure phone numbers to work with my SIP account too.

AutoPager by Wind Li

AutoPager automatically loads next pages when you reach the end of a page. It works on a ton of sites,Google,Yahoo..
It works well with most other add-ons adblock plus, WOT and most of the greasemonkey scripts.
You can also try AutoPagerLite.

DOM Inspector by Scott MacGregor

Inspect the DOM of HTML, XUL, and XML pages, including the mail...

MR Tech Toolkit by mrtech

(Formerly Local Install) The primary goal of this extension is to provide the tools needed to install and manage extensions and themes locally and tons of other features.

Note: With Toolkit 6.0 Auto Archiving is fully working in Firefox 2.0 and temp...

Console² by Philip Chee, zeniko

Console² (pronounced Console Squared or Console Two) replaces the JavaScript Console with what could be the next generation Error Console. From v0.5 includes the Console Filter extension previously available separately from the Console² website.