Making Firebug into an extension that works inside the Firefox dev tools (the same way Firebug plugins can add tabs) sounds like the best way to move forward both Firefox and Firebug.
Why? I say leave Firebug alone, or just kill it already? We don't need two inspectors.
Mozilla can't kill Firebug, even if they wanted, since it's a separate open source project and community. And the only people with two inspectors would be those that elected to have them both, by installing Firebug and using both tools.