Thanks to thecssweblog I've found some extra firefox extensions to install. The css weblog links to each extension's author page. I'm going to list them here for my convenience as links to the mozilla addons page.

MeasureIt
ColorZilla
Web Developer
FireBug
EditCSS
Cookie Crumbler
Clear Cache Button
IE View
LiveHTTPHeaders

ok, so the last one is on mozdev.org. I'd already found some of these in the past but I always forget what extensions I used to have installed when I first install firefox on a new machine.

Here's some others I've found...

Searchbar Autosizer
HTML Validator
IE Tab
Tab Mix Plus
Greasemonkey
HackBar

And a very useful greasemonkey script:
Textarea Backup