Native Developer Tools for Firefox

They are now in Firefox Aurora (pre-Beta). Also see video.

The official statement is to "Provide an alternative for most developers but leave Firebug power-users on Firebug."

Reading between the lines a little, they know there is a decent chance Firebug will start heading downhill. So they've made the decision to bring developer tools in-house and not rely entirely on that third-party.

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Native Developer Tools for Firefox is a post from CSS-Tricks