Lifehacker's "Better" add-ons, compiling multiple Greasemonkey tweaks for YouTube, Reader, Gmail, Google Calendar, Flickr, and other sites.
Handles your pictures, music, videos, and other media better than the defaults.
Add-ons that add Chrome functionality to Firefox. Based on: http://lifehacker.com/5044518/enable-chromes-best-features-in-firefox
Get more done on the web while you're making it.
Tools to strip out Flash, big images, advertisements, and other web drags.
Senior Editor Kevin Purdy's personal stash of Firefox add-ons.
Ten broadly useful extensions, as detailed at: http://bit.ly/10Xxc (There are technically 10, but Gears can't be installed from Mozilla's site)