They will end up trying to have their cake and eat it, taking money from both places. And after news like this one, many users would not want to support them, understandably.
They lost a big chunk of valuable (but not financially) user trust. It's really tricky to regain it, they would have to take a leap of faith, do "the right thing", and hope users will believe.
Or maybe we need to fork it and crowdfund (crowdhire?) firefox developers. And hope they will accept the risk of financial difficulties down the line. And there will be a long term trust/authority problem to solve - who makes decisions? But I would trust their devs personally, I think
I personally think that Firefox Pro could offer a shiny icon that says "Pro" in it, and the satisfaction of knowing that I supported a project I care about.
For an annual license, perhaps a sheet of stickers?
Cloud sync of plugins/tabs. Pocket, filesharing between devices, their VPN nobody uses, Firefox monitor integration, earlier updates (i.e. you can still get the latest build by building from source, but prebuilt libraries first get supplied to Firefox pro),...
Basically all the stuff they have acquired over the years, with few users but large support requirements.
These are just the ideas I came up with in the spot
Let's take YouTube Premium as an example to set a baseline. About 3% of users convert (50M vs 1.5B), for basically the same benefit.
Firefox starts with 200M users now and if 3% get Pro that is 6M users paying $5/month (?) for $30M/month and $360M/year in revenue. That is almost exactly that same as what Firefox makes from Google. The added benefit is that this change could transition it into a user-centric mindset.
I make basically all my income via Firefox indirectly.. if you want a slice just ask for it, don't be creepy