I have been using FF since it was almost of Beta.    I never had had a virus on a PC I own/have ever owned.   I watch You Tube videos, use FB and have never had an issue.    I think all of it comes down to: "Should I have clicked that link?".    
Ripley's picture on the Cookie Management is spot on.   I did that a few years ago and I had to set aside some time to get things setup how I like it.