Web Developer – Firefox Add-on
The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools. It is designed for Firefox, Flock and Seamonkey, and will run on any platform that these browsers support including Windows, Mac OS X and Linux.
Download Web Developer firefox add-on from here
Screenshot
how it is different from FireBug?
It was around long before FireBug. It’s similar in many ways, but does give you some extra tools that may be useful for layout and design.