Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
I don’t normally link to simple Firefox extensions, but this one caught my eye and could be beneficial for any web developer out there. It gives you very quick access to crucial information regarding any website and presents it in a clean and clever manner in order to get whatever you need done, done quick. I have used Web Developer for quite a while, but I may very well make the move to Firebug.