I would say make Firefox as light and fast as possible by keeping ALL developer tools as addons. Most users aren't going to need them at all, so it's just eating up space for no reason. I personally prefer Firebug over Inspector, so I would rather see you throw more resources into integrating it fully with FF.
The developer tools don't slow down Firefox if not used (lazy loaded), and the space used is very small. Firebug allows you to disable the builtin Inspector button.
I believe the parent was saying it should be a joint effort and all the resources should be pooled into one. I prefer Firebug as well and would love to see it get better, faster.