Proton and Mozilla are both great companies, so this is a cool partnership. But I use ProtonMail every day with a custom domain, and that tier of service comes with ProtonVPN at no added cost, so I would effectively be paying twice for the same thing to benefit Mozilla. Not unlike me, though, since I think I still have an active Pocket subscription which I very seldom use or even think about; it just quietly debits my bank account every other paycheck, but that's okay because Mozilla is pretty baller. I consider that subscription to be a monthly donation.
All things considered, I don't see existing ProtonMail users like myself to be the ideal customers here. More likely just donate outright on a monthly basis, or perhaps buy some merchandise. Proton is also a very good company in terms of what they add to the field, so changing my active subscription to ProtonMail in order to benefit Mozilla wouldn't really do much for me as a giver.