Hard to say with the limited information you gave. Firefox is a web browser and not a antivirus scanner. If these are by small popups on the bottom right corner of Firefox windows then this could just be web push notifications from sites you have allowed to send you notifications. Instead of useful information some may send you junk, spam, fake virus/trojan alerts etc. You can revoke these sites from sending you notifications as shown in second kb link below. 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