Firebug
Best. Web. Development. Tool. Ever. Just install it.
Firecookie
This add on to Firebug allows you to view, manipulate, and delete cookies. This really should just be in Firebug, anyway.
Screen Capture Elite
Very nice screen shot tool. It can also do full page screen shots.
Adblock Plus
As a developer for a company that gets it’s revenue primarily from ads, I don’t encourage you to use an ad blocker. However, sometimes I just need them all to go away while I test some JavaScript.
Xmarks Sync
Bookmark syncing across computers and browsers. I personally use it to sync my bookmarks between Firefox and Chrome.
Html Validator
Displays validation errors for any page when you view source.
