I'm getting pop-ups from a fake McAfee site how do I go about blocking them?

Are these as small popups on bottom right corner of Firefox window? If so then these are just web push notifications from sites you have allowed to send you notifications and you can revoke these sites. https://support.mozilla.org/en-US/kb/push-notifications-firefox#w_how-do-i-revoke-the-notifications-permission-for-a-specific-site Otherwise there is https://www.mcafee.com/support/s/article/000001899?language=en_US