The vast majority of users don't need it. If they want to make it native, simply bundle it with a dev version. As useful as firebug is, more FF users would probably benefit from a button that converts a page from English to Spanish (I'm aware Chrome already does this). I think they should just put out the most focussed product possible, and allow their healthy dev community to add any functionality missing.

That said, "packages" might be a good idea. The "paranoid package" could bundle adblock, noscript, trackmenot, and foxyproxy. The "developer package" could bundle firebug and anything else that might be useful. Offering binaries with these packages pre-installed from a visible link on the main download page might not be a bad idea.