My biggest complaint about both Firebug and the FF Dev Tools is the way they list downloaded content files. FF Dev Tools seems to only have a flat list of filenames, which is ridiculous. Firebug has a dropdown that can be filtered by name, which helps, but is slow.
I'd always been a Firebug user, but I finally gave Chrome Dev Tools a shot a few months back and was sold by a combination of its speed and general usability. In this case, stuff like having an actual tree of content files that I can expand.
Also, to echo another comment, Firebug seems to go haywire every so often while trying to set breakpoints. Either the breakpoint symbol just spins and never actually sets, or when you reload the page it acts like you set the breakpoint on the last line of a module, etc.
Haven't yet tried out Firebug 3, as it requires FF35+.