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.

Also worth pointing out that the built in tools have a Firebug theme in the settings.