I would NEVER use Firefox's VPN. Ever.

It boils down to trust. A VPN is only as safe as the company who runs it is honest.

There is a very big debate in the online privacy community about VPNs. Serious professionals who really care about privacy don't trust any of them. I have a couple I use. Not all VPNs are created equal by a long shot. I don't use them for anonymity. I only use them to avoid doxing.