Are we agreed that Mozilla needs a mobile platform of some kind (or a partnership) in order to ensure Firefox is the browser on it? (be it FxOS, MozDroid, or something else entirely)
Re: "FxOS, success or failure, pushes the Firefox product as a whole further," I don't see how FxOS is anything other than a waste of resources if it fails.
If Mozilla went the route of rebranding Android, they needn't fork it entirely. They could just apply their branding & app store on top of Google's releases. The app store is a necessary component because Google won't allow the use of theirs (at least without a price), and it's a user expectation of any mobile OS. (even FxOS has an app store; marketplace.firefox.com)
It's unrealistic to expect Mozilla would become a dominant mobile platform in the foreseeable future, no matter what they do. Like Microsoft, they just need to have some territory, for now. (perhaps they should also consider partnerships with "the new Microsoft", Yahoo, Amazon, Facebook, etc.)