How to extend Firebug's inspector feature?

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?

Solution

The existing Firebug 2 is broken once e10s is enabled by default in Firefox. That's why Firebug 3 (aka Firebug.next) was recreated from scratch based on the built-in devtools.

Therefore it is recommended to base new developer extensions on the built-in devtools instead of the old Firebug 2 source.
A guide describing how to do that is available at the Mozilla Developer Network (MDN). And there are also example extensions in the Firebug repository on GitHub as well as a template extension created by one of the devtools developers.