You should implement such extension on top of native developer tools in Firefox.
Not as an extension to Firebug 2 - it'll stop working as soon as e10s (multi process
browser support) is enabled in Firefox (release channel).
Here is a repo with bunch of examples showing how to make extensions
for native developer tools:
https://github.com/firebug/devtools-extension-examplesIf you missing an example showing specific features
let me know. I am happy to provide it.
Honza