Adblock Plus - Custom filters for removing ads and privacy invaders
Chatzilla - The simplest IRC client built-in to Firefox.
Foxmarks - Bookmarks synchronizer across multiple computers. Easy and secure!
Video DownloadHelper - Help download embedded videos and other files you clicked on
Evernote Web Clipper - Browser integration with
Evernote
OpenDownload - Be able to "run" .exe straight out of the browser without saving.
Right-Click Link - Open any link by right clicking and selecting to open. Works for all links with a protocol in front (HTTP, IRC, etc)
Update Scanner - Check and monitor for changes in websites. Useful if no RSS feeds are available.
Nightly Tester Tools - Excellent set of tools to test builds and betas of Firefox. Includes addon version compatibility override and immediate addon installation.
MR Tech Local Install - The must have complement of Nightly Tester Tools. Adds all sorts of customization options in about
:config and restart option.