Firefox doesn't advertise or send popup notice of such kinds. Are you sure you didn't get a malware infection spamming it's Mcafee impersonation. You need to use Firefox started in Private Mode browsing and check there isn't malware Addons causing the fake notification.

''<p>w.st26 [[#question-1409709|said]]</p>'' <blockquote>For the past week or two I have been getting these annoying pop ups with the Mozilla Firebox emblem demanding that I download the Mcafee and scan my computer for viruses. I don't want to have anything to do with Mcafee or download ANYTHING from them. What's going on? How do I fix this? </blockquote> The company Mozilla does not have any affiliation with any antivirus companies. Mozilla also does not prompt Firefox users to download stuff from random site urls and just because it has a Firefox emblem on page does not mean it is official. It could be a form of malvertising ad you encountered https://www.imperva.com/learn/application-security/malvertising/ This malvertising ads thing was a [https://support.mozilla.org/en-US/forums/contributors/712056 huge headache some years ago] when there was a long list of fake Firefox updates sites (were only up for a day or two) that some Firefox users encountered in malvertising ads. Chrome users were targeted with fake updates (for malware) sites in a similar fashion also at same time then. Since you use Windows, and not macOS or Linux there is also the possibility you could be infected in some way. https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware