I think that Firefox has lagged behind because of Firebug. When there is already a way for developers to do things, why invent your own, unless you can make it better?
Do I need to do any web dev work? Just do it in firebug.
Do I need to drastically change my browser's behaviour? There's probably a setting for that in the web dev toolbar.
Do I want to do anything that involves monitoring or changing the browser itself? check the FF web dev tools.
Between the three, I don't understand how people can claim chrome has a superior toolset.