Are these small popups in bottom corner of Firefox windows? if so then this sounds like you may be getting web push notifications from one or more sites that you have allowed. Unfortunately sometimes these can be fake virus alerts from some sites. You can revoke permissions. 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