I can understand offering something for free temporarily in order to encourage user conversion later from people that need more than 50 GB of data (hey, a way Mozilla can make money from its users!).

But, confusingly, paid Mozilla VPN and free Firefox VPN are two different things. After the free trial expires, you’d have to disable the built-in, unremovable Firefox VPN, then install a separate Mozilla VPN extension which does nearly the same thing but with Mullvad servers instead of Fastly ones.