Interesting. I'd actually pay for an in browser VPN, it's handy to be able to switch countries on the fly. Ideally even per browser tab.
I would not however pay for Mozilla's mullvad thing. I don't like mullvad since they dropped port forwarding and OpenVPN. I use proton now for that. But in the browser is a different usecase for me.
It's just weird that it's not possible to pay for this but only for the thing I don't want.