I have no idea what Mozilla has been building, but I would guess building a browser from scratch with development in mind gives more possibilities than the most advanced extension for a browser. As long as the rendering engine remains the same as Firefox it wouldn't technically be a "different" browser, just a developer flavoured Firefox.

I think it's great that Mozilla is exploring new territories; it may or may not turn out to be a good idea, but give them the benefit of the doubt, at the end of the day no matter how good web developers tools currently are, without experimentation there would eventually be stagnation.