I have found out that Firebug for Firefox is an open source project and we can extend it.
By searching for documentaion about Firebug extension development I found the following website:
http://www.softwareishard.com/blog/extending-firebug/
Unfortunately its petty old and doesn't have information for current versions (i.e. 2.0.x) of Firebug.
My goal is to extend Firebug's inspector feature.
So can anyone please tell where I can find an up-to-date guide for Firebug extension development?