Are these as small popups on bottom right corner of Firefox windows?. If so then these may just be web push notifications from sites you have allowed to send you notifications. Instead of usefull information some may send you junk, spam, fake virus/trojan alerts etc. You can revoke these sites from sending you notifications. 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