When you enter debug mode in SpiderMonkey most (all? I forget) jit'ing and optimizations won't happen; code becomes a lot slower. The new debugging api allows you to debug specific tabs, while the old one would force all of the browser (including the chrome js that handles the browser's UI) to enter debug mode and the whole browser gets slow.
Because Honza is working on porting firebug to the new debugger API, it will get faster, and so will all of firefox when you are using firebug.
IIRC, Honza is not just working on porting from the old api to the new one, but also on top of our remote debugging protocol. This means that we improve the debugging server, and both the native tools and firebug get better. Plus, you'll be able to use firebug on your laptop, connected remotely to firefox on your android phone, or firefox os.
Not really, no. There was some talk about adding a Firebug panel to the built-in tools, but nothing has really come from it, and any deeper UI integration is probably out of the question for being too much work. But suggestions about how to make it better are still welcome, of course.
The main thing we have gotten out of the new devtools are new cool APIs, e.g. for debugging, paint flashing, CSS auto-completion, profiling (though we haven't gotten around to using most of them yet).
Because Honza is working on porting firebug to the new debugger API, it will get faster, and so will all of firefox when you are using firebug.
IIRC, Honza is not just working on porting from the old api to the new one, but also on top of our remote debugging protocol. This means that we improve the debugging server, and both the native tools and firebug get better. Plus, you'll be able to use firebug on your laptop, connected remotely to firefox on your android phone, or firefox os.