Firebug is a free and open-source web browser extension for Mozilla Firefox that facilitates the live debugging, editing, and monitoring of any website's CSS, HTML, DOM, XHR, and JavaScript.Firebug is licensed under the BSD license and was initially written in January 2006 by Joe Hewitt, one of the original Firefox creators. The Firebug Working Group oversees the open source development and extension of Firebug.