That's not a legit warning, that's a malware scam. You need to get your computer properly scanned to remove that infections or whatever site is showing that malware scam.

I know but I ran Windows Security "Full scan" (took 4.2 hours) and then, when the threats continued to appear, I ran "Microsoft Defender Offline scan" which took about 1/2 hour... ...and I just got 10 MORE threats.

Thank you for that information but WHAT scan will remove them? Thanks! John

Your screenshot shows these are just Web Push Notifications from sites you have allowed to send you notifications. Instead of useful information, some may instead send you junk, spam or fake virus/trojan notification alerts to try and get you infected. You can revoke these sites from sending you anymore 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