Well, I don't really work with CSS much, but js.
Are you consider the debugging tools well done as well? And do you know chrome dev tools? With its elaborate performance tools etc?
I currently work more with CSS so I find the debugging tools on both good enough for my purposes. I do know the chrome dev tools and find them to be good, just happen to prefer the FF dev editions ease with layout.
I mostly use Console, Network and Inspector tabs and I rarely debug (in general.)
Especially Network tab in Firefox's dev tools is much superior to Chrome's one, IMHO.
