Good Developer Browser Extensions

A co-worker points me to his favorite list of Firefox browser extensions, I figured I'd pass it on:

Live HTTP Headers - http://livehttpheaders.mozdev.org/
Add & Edit Cookies - http://addneditcookies.mozdev.org/
MeasureIt 0.2.5 - http://www.kevinfreitas.net/extensions/measureit/
User Agent Switcher - http://chrispederick.com/work/firefox/useragentswitcher/

Thanks Bell :D