I don't get it. Who are they outsourcing it to?
I’ve been using Mullvad for several years now and they’ve been reliable. They have independent security audits done and are known for their privacy.
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.