Firebug isn't 'owned' by Mozilla, and hasn't ever been, however Mozilla has contributed financially to it's development for some years, and still is.

In addition to supporting Firebug, we're also experimenting with alternative developer tools.

It's my belief that most developer tools just present the user with the information that was given to the browser in the first place, and that we can do a lot to explain how the browser actually understands all that stuff. The native developer tools are a step in that direction.

I work on Mozilla developer tools.