I'm not sure that anyone has a definative answer to that question right now. The new tools are interesting because they do things in a different way, but there is a lot they don't do, so we're supporting both.

It's possible that one day Firebug will be totally superceeded. It's possible that both tools will have separate audiences for some time. It's possible that the new native tools will be a total flop (although I have a vested interest in making sure that doesn't happen!)