I think FF was pushing it in a recent update. What a piece of crap. Ranked 77th of 78th.
Mozilla has its own VPN service. It’s unrelated to the one you were using.
Warning: Potential Security Risk Ahead
Firefox detected a potential security threat and did not continue to thebestvpn.com. If you visit this site, attackers could try to steal information like your passwords, emails, or credit card details.
What can you do about it?
The issue is most likely with the website, and there is nothing you can do to resolve it.
If you are on a corporate network or using anti-virus software, you can reach out to the support teams for assistance. You can also notify the website’s administrator about the problem.