> I fully expect that this move will certainly lose Mozilla a few Firefox users, but on balance it'll be a net win
I certainly hope so, for the sake of an open web. Hope they can take on the universal pushing and bundling that Google does with Chrome. Anecdotally I can say that none of my acquaintances are even aware of Firefox because Chrome can with their OS and they never saw a need to look beyond it. I hope Mozilla has a clear strategy to advocate and market Firefox, since technical merit alone is not going to out-compete Chrome's entrenched position.
I fully expect that this move will certainly lose Mozilla a few Firefox users, but on balance it'll be a net win: they were already losing users to performance problems and lack of modern security features. The users the lose due to lost extensions will be tiny compared to the users they won't lose in the future now that Firefox actually feels like a modern browser performance- and security-wise.
I expect Mozilla knows a ton more about their users than we do and any guesses about user retention due to this move are pure speculation (including what I wrote above). I trust them to do the right thing here, a trust they've earned over the past (nearly) two decades that I do not place in, say, Google & Chrome.