Are these popups in the bottom right corner of Firefox window? if so then these may just be web push notifications from sites you have allowed to send you notifications. instead of useful information some may send you f=junk, spam, fake virus/trojan alerts, even fake antivirus client alerts etc. You can revoke the 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