It is mostly applied for debugging web pages, security testing and performance testing of web software products.
The changes come into operation immediately. It monitors the process of a web page loading, shows HTTP-headers and AJAX-requests.
Firebug functionality can be extended by a number of utilities, they include Firecookie, FirePHP, FireRainbow, YSlow.
Firebug is free and open-source, it is developed by Joe Hewitt.
Web Application Testing
User Interface Testing