The Facebook "phone" approach is interesting: take over the phone to such a degree that it delivers a FB "experience". No dependencies of the Evil Triangle of the cell world: Handset mfgr, OS provider, Carrier.
Yet Moz apparently is looking to become another OS provider? I'm wondering if they might take the FB approach instead.
FB will succeed now with Android, there appears to be no App Store issues. But my bet they'll also force Apple to cry Uncle! relatively soon. Now THAT would be progress
We have a launcher-based approach in the works too. IMHO it's not going to sweep the world, and neither is FB Home.
Strategically, the big problem all such approaches face remains, and it may even get worse as Android matures. Google owns that OS and controls its APIs and rules for extensions.
Anyway, Android 4 doesn't even fit on 256M phones single-core phones, where Firefox OS is entering the market. It is priced out of the launch countries.
So taking a high-end-focused Android-only approach of "[do an FB-Home-like launcher] instead" does not make sense tactically or strategically.
"Also" rather than "instead" can help, we're exploring in the context of Firefox for Android and the Web Runtime based on it -- but it's not exclusively & clearly winning such that we would bet Mozilla on it.