Firebug is an amazing web development extension for Firefox. Anyone who does any web development should use it.
The only downside so far is that it clashes with one of my other favorite extensions, HTML Validator—if both are enabled, I can’t type anything into form fields on a page without ‘find as you type’ taking over and grabbing all the input. I’m hoping this will be fixed soon, but even if it remains forever, both extensions make it worth the trouble of disabling/enabling and restarting Firefox.
If you do web development, go get Firebug.