Firebug is a plug-in for the Firefox web broswer that functions as a web development tool. Users can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page. Firebug is free and open source, released under the BSD license. 

It can also be used for website security/penetration testing, as well as a performance monitor that analyzes a particular site’s bandwidth usage.

Firebug is available at the project homepage or the Mozilla addons page.