On the other hand Nexo did A and A+ security rating by some other organizations, you can see this by looking at tab “Third-party test” on the same page where Mozzila Observatory test is. As I see it security ratings are very individual for different security organizations with different stress points.
What makes my mind more relaxed is that they are secured with cold wallets from BitGo and also have insurance with Lloyd’s. Sure is not optimal that Nexo is basically centralized entity, but they are basically owned by Credissimo, which has a great and long record track as a leading European FinTech Group founded in 2007.
Always there are good and bad things, good one is surely that they are in between very few projects in crypto creating fantastic revenue and they are dividing part of it to us investors as dividens.