https://support.mozilla.org/en-US/kb/push-notifications-firefox https://support.mozilla.org/en-US/kb/push-notifications-firefox#w_how-do-i-revoke-web-push-permissions-for-a-specific-site Are these small popups on bottom right corner of Firefox window? if so then they may be web push notifications from site(s) you have allowed to send you notifications. Unfortunately sometimes they can be fake virus or fake antivirus (like mcafee) claiming you are infected instead of useful notifications from the sites. The fact that is says "PC is virus infected" shows it is fake when you have a Mac. Normally PC is in reference to a Windows system. The second link shows how to revoke those sites from sending you more notifications.