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.

That’s extremely weird, I’ve never heard of Firefox not letting you browse until you update. When snap auto-updates Firefox there’s usually a notification bubble asking to close your browser to update but you can dismiss it and keep browsing in my experience.

The hate for snap on Linux forums always felt weird to me, I’ve literally never had issues with Firefox snap. I understand being frustrated with it on the principle that it feels Windows-y to force it on the system, but the Firefox snap is packaged by Mozilla and bundles the latest Mesa libraries instead of using the older libraries from the Debian repos that don’t have the latest performance fixes, so its also faster than installing through .deb. And Mozilla has Debian repos for Firefox you can add to your sources.list if you really insist. There’s also nothing preventing you from installing Flatpak and using that on Ubuntu.