It appears that some users have encountered the firefox Xp antivirus error message. This issue occurs due to a number of factors. Let’s look at them now. Norton 360 also has a useful Firefox extension that securely sells passwords for various websites a person uses. But in my experience, I have Avast (which I use at home) and Avira AntiVir (at work) – 2 main free and good antivirus plans.