how to block mcfee popups in firefox how to block ads in firefox

Are these small popups only on the far right corner of Firefox windows? if so then this may just be web push notifications sent from sites you have allowed to send you notifications from. You can revoke these 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

Unfortunately some sites may send you junk, spam, fake virus alerts (including fake McAfee alerts) etc instead of useful information notifications.

IT covers the whole screen It takes three lots of closing screens to get rid of it It then comes back later It is a Mcfee anti virus ad

Sounds like a fake and you need to find out what site is doing this or if your system is infected from a computer shop.

IT covers the whole screen It takes three lots of closing screens to get rid of it It then comes back later It is a Mcfee anti virus ad

Does it look like the screenshot below? as it was taken from /questions/1490578

See also https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware