Are these small popups on bottom corner? if so then you may be getting Web Push Notifications from sites you have allowed notifications from. Fake McAfee and virus alerts popups can occur from some sites. You can revoke notifications from any site you do not want. https://support.mozilla.org/en-US/kb/push-notifications-firefox#w_how-do-i-revoke-web-push-permissions-for-a-specific-site