As I understand it (it's still somewhat in the planning stages, and things are evolving), Firefox will ship with a set of developer tools which will cover the major use cases (inspector, console, etc). Firebug will be an extension, and will add more advanced functionality. It's being rewritten in JavaScript, and could potentially work on any browser (Firebug and Firebug Lite are going to merge).
This is how I understood it when I talked to the Firebug team a few months ago. Mozilla has since hired a ton of people for their Dev Tools team and John J Barton has left Firebug due to lack of funding from IBM -- so I have no clue if things have since changed.
PLEASE integrate these things carefully and in a sensible way. If I have to use "dev tools" for x, b, and z and firebug for a, y and c that's going to be annoying as hell.
This is how I understood it when I talked to the Firebug team a few months ago. Mozilla has since hired a ton of people for their Dev Tools team and John J Barton has left Firebug due to lack of funding from IBM -- so I have no clue if things have since changed.