When Mozilla ships better developer tools than Chrome, I'm sure there will be posts just like this one. The fact that developers prefer Chrome is not a failing of developers; it's a failing of Mozilla to provide compelling tools.
I don't know much, but I don't understand Mozilla's effort to reimplement firebug natively. Right now their solution seems unusable and far beyond firebug.
They're reimplementing it because Firebug is an unmaintained mess.
Mozilla is just in an unfortunate situation, where Firebug's ground-breaking functionality was duplicated as a first-party feature of other browsers. So now as Chrome Inspector continues to add more and more deep functionality, Firebug is hampered both by being third party code and subsisting on Firefox's addon API.