Good luck to them, assuming they stick to their principles. But IIRC, experiments with paid browsers have typically not ended well. What makes them think they can pull it off?
- it's only an experimental additional source of revenue. They already got money covered, they are just trying to find new way to become more financially independent.
- they don't intend to sell the current state of Firefox. It will still be free. They intend to add stuff on top of the already awesome experience.
1. Paying users are too valuable.
2. Free users can "opt out" of experiments by paying - and they'll get some nice features for "free" as they do so.
(Edit, formatting)