Mozilla does not push any form of antivirus clients on Firefox users whether on Windows, macOS, or Linux. Perhaps you are referring to small popups on bottom right corner as they can be fake McAfee notifications from sites you have allowed to send 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 Or if this is a larger popup more in middle of Firefox window and not on bottom right corner, see https://www.mcafee.com/support/s/article/000001899?language=en_US

This McAfee is a full screen after FireFox says a site open a bad link. It only come when another full screen comes up from FireFox asking me to prove I am not a robot...