Just installed Firefox 1.0.1 for OS X tonight. It doesn’t seem to handle migrating from previous version perfectly, so I’m updating my bookmarks + extensions. This is one of the excellent ones that I’ve come to rely upon.

Mozilla Update :: Extensions — More Info: Web Developer

Web Developer 0.8

By Chris Pederick
Adds a menu and a toolbar to the browser with various web developer tools. Current tools include converting form POSTs to GETs, hiding images, outlining block level elements, and disabling styles – plus many more.