That's not a Firefox caused but either your A/V is infected or doesn't work or your Addon is the Virus that is loading the popup. But by default Firefox doesn't send you popup beyond the updates notifications.

^ignore the post above. ''<p>drvansteiner [[#question-1475582|said]]</p>'' <blockquote>A continuous stream of "antivirus" via tomp3.cc popups started appearing today on my Win 10 desktop. Although there is a "Dismiss" button, once pushed the popups return immediately </blockquote> Those look like Web Push Notifications from sites you have allowed to send you notifications from. Unfortunately some sites may send you spam, junk or fake virus alerts instead of useful information. You can revoke those sites. 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