Some great Firefox extensions:
EditCSS - Edit your CSS on the fly.
Firebug - Explore the DOM.
MeasureIt - Measure pixel distances.
A big list of Extensions
ColorZilla - Find the color of anything on a webpage.
LiveHTTPHeaders - Watch HTTP headers as they are downloaded.
Aardvark - See how a page is constructed, block by block.
Web Developer Extension - Lots of good stuff for web developers.
Firefox tips and tricks