Are these small popups on bottom right corner of Firefox windows?. 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 instead send you junk, spa, fake virus/trojan alerts, fake antivirus alerts like say fake McAfee alerts. You can revoke 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

Hi James, no it's taking over the majority of my screen. All I can do is click on the "x" in the corner and then twice again to get rid of their ad

Hmm I was hoping it was not that as I have seen a few threads about this in the last couple months and a cause of this was not found yet.

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

Sounds like a job for Sysinternals Process Explorer

https://www.youtube.com/watch?v=y2bNLCWHFNs https://www.youtube.com/results?search_query=process+explorer+bullseye

Hello,

Your browser or Windows system displays fake McAfee notifications https://www.mcafee.com/support/s/article/000001899?language=en_US

Click the +sign on How to fix fake PC App Store pop-ups