Because your competitors are free and users expect free and we have a culture stuck in a rut of "everything is free" because ads don't have them taking money out of their wallet.
Not every software is in a niche where they can flip the switch back into a premium model and have a good outcome. If Firefox did it, most of us would stop using it.
Maybe Firefox is at a point where that's a boat worth rocking, but it certainly isn't obvious nor without fatal risk.
The best route still appears to be paying for whatever paid services Mozilla comes out with (VPN, Pocket, ...?) even if you don't use them. I don't know if a "pay what you like" option would be problematic?
(Regulators watch pretty closely when you have 100s of millions of dollars of annual income.)
Examples: Pocket and their new VPN service.
Sure, people with business interests will try to push the idea of what constitutes a valid option in various directions, but as someone more thoughtful than me once said, "prediction is hard, especially if it's about the future," and as another thoughtful person said, "even The Future has a future."
I think subscriptions to software can do a lot to bring in line users needs with company needs. Especially around security of purchased software - if there are no profits to be made as the software is already sold why patch security issues or bug fixes, your only goal is to sell the next one.
Plus, as the other commenter says, I am not going to recommend Firefox to my non-technical friends if it requires non-trivial amount of money.
