Are these small popups on bottom corner? if so then these may actually just be Web Push Notifications from one or more websites you have allowed to send you notifications from. Unfortunately some of the notifications from some sites can be fake virus and fake random Antivirus client alerts. You can revoke permissions for the sites. 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