Firefox is a web browser and not some form of antivirus or malware scanner. Are these small popups on the bottom right corner of Firefox windows?. If so then these may just be web push notifications from sites you have allowed to send you notifications. Instead of being useful information, some sites may just send you junk, spam and fake virus/trojan alerts. You can revoke these sites from sending you notifications. 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