That sounds like a scam. I would hang up on them. If you are concerned that your computer might be infected, you could try some of the programs in our support article: [[Troubleshoot Firefox issues caused by malware]]. Those are free and take considerable time to run. If they finish quickly and especially if they demand a payment, you may have a serious problem and the specialized forums listed in the article would be a good place to seek help with that.

Scam ! ! !