That is not true. I am sure add-ons can access the same stuff that built-in tools can. Most of the stuff are exposed as JS objects and there are APIs for a lot of stuff. APIs for devtools may be a bit hard to find the documentation sometimes. I guess it is because it is still being developed.
For me Firebug seems ~2x as slow (eg, for a 1s operation in native tools, firebug takes 2s). Opening Firebug on my workstation causes a noticable pause in all activity on page for several seconds, while opening the native tools only takes about half a second. All of Firefox's dev tools are still a ways from being as fast and bug-free as the Webkit tools (both Chrome and Safari). I can't cite specific examples since I dropped Firebug in Firefox once the native tools reached a certain level of quality. I just suggest trying to use 'em and seeing if you like them more than Firebug. I did.
I have a little bit of a hard time believing this: Firefox Addons can use almost all the APIs in the browser that built-in code can, and I'm at least led to believe the new debugging APIs the built-in tools are using are no exception to that rule. https://developer.mozilla.org/en-US/docs/Tools/Debugger-API seems to agree on those points. Or do you mean the tools themselves aren't extensible?
It's been a few years since I've written any Firefox code or touched XPCOM, so I could be looking on this without up-to-date information: I'd be interested if the Firebug authors or others have written any thoughts on struggles they have with this.
I don't recommend using it any longer...