As far as I can see, the only thing they've "added" was their own client applications. The connection limits and server locations are the same as Mullvad. One

As far as I can see, the only thing they've "added" was their own client applications. The connection limits and server locations are the same as Mullvad. One thing that Mozilla does not offer yet, but Mullvad does, are Wireguard proxies. These are socks5 proxies run internally on every Wireguard server. Using these you can connect to another server, for example EU --> US, and achieve "multihopping". Mozilla also seems to only offer the Wireguard protocol, but this doesn't seem like a big deal to me.