This release is what they termed MVP. Everything after it is post-MVP. It means minimum viable product. In my personal opinion, a lot more functionality was included than what most would call an MVP.

The big flash announcement is to get early adopters for testing. There's no reason to delay testing until the MVP is ready, as long as testers remain aware that Firefox Preview is a preview that shouldn't be expected to already have all features implemented.

Yes, that's what "viable" means. Viable for big flashy announcement.