Are you visiting or have you ever visited that site? If not, then you are infected and need to run some scans.
If you get it again, you'll need to scan with with other programs besides your antivirus.
ADWcleaner
Junkware tool removal tool
Malwarebytes
He said it came up while Firefox was running. So it's possible he has a bad add-on that got installed.I would still scan the system especially if web browser isn't open and getting this message. Never had this on my system but have seen it on my clients many times. Always scanned and found crap, was always able to get rid of it.
He said it came up while Firefox was running. So it's possible he has a bad add-on that got installed.