The problem is, it's pretty useless without Firebug compared to Chrome, at least for me -- much of my browser use is for web development.
(this may change with the built-in debug tools set for Firefox 10.0 -- let us hope)
I'm also a bit perturbed by the fact that Mozilla has always touted extensions as a key feature of Firefox... only to say you should disable them all if you want good performance.
A big part of the built-in tools isn't to replace Firebug. It's to provide APIs for Firebug (and other addons!) to be able to do what they do even better.
Addons don't in general hurt perf, but it is definitely possible for them to. Firebug, in particular, disables the JIT so it can debug it, making Javascript execution painfully slow, and thus the browser is slow too, since large parts are written in JS. Firebug isn't to blame though; we just haven't provided them with APIs to do better yet.
I'm also a bit perturbed by the fact that Mozilla has always touted extensions as a key feature of Firefox... only to say you should disable them all if you want good performance.