Are these small popups on bottom right corner of Firefox windows?. If so then this sounds like web push notifications from sites you have allowed to send you notifications. Instead of useful information some sites may send you junk, spam, fake virus/trojan alerts etc. You can revoke these sites from sending you notifications as shown in second kb link below. 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