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!)
Bringing the new functionality into Firefox, where it can be more exhaustively slimmed down and the primary team is responsible for it makes sense to me.
Heck, if the basic functionality is there then Firebug can be written to take advantage of those APIs, which would be a win all round.
I am hard working to fix all Firebug memory leaks (and did fix some of them recently). What I really need is a simple scenario showing that Firebug leaks. That helps me a lot!
How am I to tell whether leaks are from Firefox or Firebug? What I know is that after a full day developing Firefox will use 800MB of RAM, and the only way to get rid of that is to close the entire process. I use Firebug heavily and I know that _something_ is leaking somewhere.
That's nice to hear! I'll take a look on how to help.
As mkopinsky points out it's sometimes hard to tell what really leaks memory.
I can tell the same thing as he, but I think it has been improved a lot lately. Sometimes I had after some time working around 1.5GB Memory usage, but this is better now.
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!)