Favorite extensions to Firefox
Live HTTP Headers (http://livehttpheaders.mozdev.org/) is helpful for determining what's going on with browser/webapp interactions.
NoScript (http://www.noscript.net/whats) allows per-site control over javascript
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.