Really just that I've used Firebug for so long that using anything else is painful. I've tried switching but always ended up going back. Much of it is interface differences, but some of it is actual differences in functionality: if Firebug has something which another inspector does not, then it's super-frustrating. If another inspector has something that Firebug lacks, then I don't even notice.
Edit: I should emphasize that I'm not saying Firebug is superior, just that I'm used to it.
PSA: The Firebug you know (version 2) is incompatible to e10s and there will be no e10s-compatible Firebug 3. Instead, Firebug.next is built-into Firefox(!):
Though you're used to Firebug and find it convenient, it might be worth looking at something else once in a while to see what else is out there (especially if you're not using addons on top of Firebug to enhance it). Perhaps you could have a look at Firefox Developer Edition and try it. [1]
I would really miss the DOM tab of Firebug. You can explorer the DOM state which is very useful. Non of the other DevTools anywhere has that feature to my knowlegde.
Edit: I should emphasize that I'm not saying Firebug is superior, just that I'm used to it.