This is a quick list of Add-ons in FF that I use:
Firebug – http://getfirebug.com/
IE Tab – https://addons.mozilla.org/en-US/firefox/addon/1419
MeasureIt -http://www.kevinfreitas.net/extensions/measureit/
ColorZilla – http://www.colorzilla.com/firefox/
Resizeable textarea – https://addons.mozilla.org/en-US/firefox/addon/3818
Web Developer – http://chrispederick.com/work/web-developer/
HTML Validator – http://users.skynet.be/mgueury/mozilla/
CSS Validator – http://www.nu22.com/firefox/cssvalidator/
Operator – http://www.kaply.com/weblog/operator/
Tails Export – https://addons.mozilla.org/en-US/firefox/addon/2240?id=2240
ShowIP – http://code.google.com/p/firefox-showip/
YSlow – http://developer.yahoo.com/yslow/