I got a similar question when someone asked me at a short firefox development tools demo I gave: when would I use firebug over native development tools? I didn't have a good answer. (they had noted I had firebug installed)

I liked firbugs javascript debugger a bit better. Firefox has been updating the tools alot lately though and I like them. Though sometimes they tend to flake out (javascript debugger blowing over breakpoints until restart...).

I'll give the new firebug a try.

The last thing that still bothers me (now that Firefox 30 added margin/padding highlighting) is that the built in dev tools don't have a way to manage cookies.

I'm in the same boat, but I run into weird bugs.. like dev-tools inside my dev-tools, or some sort of split screen that I can't get rid of. I just restart them, but still weird

Ever since FF tools added the "clear" buttons in the Console and Network tab, they won me over

I like the built-in tools, but the two things I like in Firebug and use extensively are the DOM inspector and pretty printing JSON results in the Net panel for API calls.