Useful FireFox extensions for Dev & Debug

• Firebug - The #1 dev/debug tool for web work, period.

• Flashbug - Flashbug is an extension that is built on top of Firebug that lets you see trace logs coming from a swf, among other things.

It goes without saying that if you do any amount of web work that involves server-side communication, that you should have a full-fledged HTTP debugging proxy.  The one I use is called Charles and I highly recommend it.