Whenever you are surfing any site and it says you have virus please install this this software that only a way to promote there software and services. If your application doesn't show any viruses then there is no virus present in your phone. Don't listen to those ad's.

I suggest also checking the following:

(1) Disable or remove any unknown add-ons from your Firefox

See: Uninstall Firefox for Android extensions

(2) If you do not already have a security app for your phone, consider using one or both of the following, or another security app that looks good, which you can install through the Play store app:

• Lookout: https://play.google.com/store/apps/details?id=com.lookout (quieter)
• CM Security: https://play.google.com/store/apps/details?id=com.cleanmaster.security (more features, but constant notifications can be annoying)