Credit Card only it looks like. Mozilla VPN is just a wrapper around Mullvad (it uses their servers and technology) so I would recommend getting that. Mullvad accepts a lot of more bespoke payment methods (crypto, wire transfer, cash in an envelope). The main reason to get Mozilla VPN is to bundle it with Mozilla’s other services.

I’m wondering how this will affect Linux support. Steam client on Linux depends on very old 32-bit libs from Ubuntu 12.04 (!) and is a major reason for distros keeping their 32bit support