Mozilla does not do pop-ups ever for Firefox Updates.

Please ignore and read and follow asap : Sign out of anything your signed into. Read and do : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware can also run https://www.malwarebytes.com/adwcleaner/

Keep the logs and I suggest https://forums.malwarebytes.com/topic/9573-im-infected-what-do-i-do-now/ Post in 1 forum only and wait for a answer. Do not do any banking.

Please let us know if this solved your issue or if need further assistance.

This is Not due to malware on your Windows but due to some Ads that target some Windows users using Firefox.

https://support.mozilla.org/en-US/kb/i-found-fake-firefox-update

alawson1970 said

I know others are having this problem. It is very annoying. At the link below, they suggest installing an ad blocker, but it says it has to have permission to "Access your data for all websites" and other things that sound troubling. How do I know this is legitimate? Thanks https://support.mozilla.org/en-US/questions/1158554

It says :

Since you’re in control of your Firefox, the choice to grant or deny these requests is yours. Accepting permissions does not inherently compromise your browser’s performance or security, but in some rare cases risk may be involved.

Totally legit and nothing for you to worry about. It would be quite wise even if you decided to install uBlock Origin, as it might prevent these fake updates from bothering you again.