i would not have minded if mozilla had offered their own vpn service, trusting some third party sounds like a bad idea.
For better or worse, Mozilla managed to brand itself as the equivalent of the open internet and an organisation that would put the users first.
This is quite important in today's world that's full of Googles, Facebooks, Microsofts etc.
That doesn't mean that Mozilla has done nothing wrong. I'm just saying that I would feel much better having a VPN service run by Mozilla as opposed to a VPN being run by Facebook.