Favorite extensions to Firefox
Sage (http://sage.mozdev.org/) RSS reader
IE Tab (http://ietab.mozdev.org/) allows opening pages with IE inside of Firefox. This is sometimes useful for brain-dead web sites.
Web Development Tools
Firebug (http://getfirebug.com/) provides a large suite of tools.
Html Validator (http://users.skynet.be/mgueury/mozilla/)
Live HTTP Headers (http://livehttpheaders.mozdev.org/) is helpful for determining what's going on with browser/webapp interactions. It shows the headers for requests and responses.
XPather (http://xpath.alephzarro.com/) displays and evaluates xpaths for page elements.