I do expect that's the next step at Mozilla - locking features behind paywall with some premium plan. Cloud sync probably will fall into that basket. And if that eventually won't work - they'll surely announce it's time to "sunset".
Personally, I think that's what they should've been doing all along. If it doesn't work at this point, it's because it's too late, and they've already burned enough of their credibility that people don't want to give them money anymore.
I do expect that's the next step at Mozilla - locking features behind paywall with some premium plan. Cloud sync probably will fall into that basket. And if that eventually won't work - they'll surely announce it's time to "sunset".