What are your favorite Mozilla Plugin

[James Clark]

I frequently use Web Developer - it's excellent.

[Immerse]

Here's my list. They're all equally important for me:

Adblock Plus - I don't like pages with 20+ ads on them. So I block ads on most sites.
ColorZilla
Download Statusbar - groups downloads into a button for easy access
Firebug - because it's awesome
FireGestures - for easy surfing
Greasemonkey - for a couple of user scripts
Live HTTP Headers - for inspecting headers sent to and from my browser
Screengrab - for making full page screenshots
WebDeveloper Toolbar - for a couple of things missing in Firebug
HTML Validator - for checking to see how many mistakes I've made in my (X)HTML