Introducing Firefox Private Network VPN’s Official Product – the Mozilla VPN – Future Releases

Looks like they are using wireguard as the protocol and mullvad as the partner.

Seems that the VPN provider has to take care with storing your IP when using wireguard (https://restoreprivacy.com/wireguard/), which mullvad does. In addition protonvpn's thoughts on wireguard (https://protonvpn.com/blog/whats-the-best-vpn-protocol/), although its a year old.

And they'll have a client for linux and mac soon.