Having worked at Mozilla I can honestly say that I'm confident Mozilla can't keep a neferious secret :)
There is a lot of passionate privacy activists at Mozilla. Many of whom to would leak an NSL at the risk of persecution.
(In fact I dare say the lineup would be long)
By siding with you they can certainly pave over the asymmetry in the relationship between provider and users.
I trust Mozilla, not ProtonVPN, and I trust Mozilla vetted them and will continue to look them over the shoulder.
That's more than I can say for my current VPN provider.