In the same vein, but with less idiomatic C, note also https://github.com/hnsl/librcd. I had the opportunity to work with it two years ago at Jumpstarter, and it was quite wonderful.
Crashes and slowness that vary with Firefox versions is Firefox's fault, and in this case the note about tab switching points pretty clearly to a problem with the debugger service.
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).