Are these notifications as small popups on the bottom right corner of Firefox windows?. If so then these are likely web push notifications from sites you have allowed to send you notifications. Instead of useful 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