I trust Mozilla to not cooperate with anyone less powerful than Uncle Sam which is a lot more than most shady fly-by-night VPN operators.
If you're using your VPN as protection against your ISP, wifi provider (internet Cafe, school, workplace, home), or some other MITM you're better off with a service run by a serious company, with a lot to lose from a scandal and a long track record of not lying and being technically competent.
If you're looking to hide your browsing from the US government you should a) give up or b) definitely not use a commercial VPN.
1. Their Backdoor of Advertisements Plugins (Ref the USA network Ads for Mr. Robot)
2. Their Forcing DoH through CloudFlare
3. Their change to require OptOut instead of OptIn for most Privacy violating features (like User Telmentary data)
4. Their starting of the Mozilla Ministry of Truth strait out of an Orwell novel
5. Banning of Plugins due to Ideology
6. Their recent push for Online Censorship
That is just a start
https://premium.firefox.com/vpn/
Presumably, they'll partner with a vendor that they can audit, but who knows.
Having worked at Mozilla I'm confident lots of individuals would leak if forced to build backdoors :)