Are these small popups on bottom corner? if so then they may be web push notifications from sites you have allowed to send notifications. Unfortunately they may appear as fake virus alerts or McAfee or Norton alerts or such from some sites. You can revoke these 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