McAfee???

Are these small popups on bottom right corner of Firefox windows?. If so then they may just be web push notifications from sites you have allowed to send you notifications from. Unfortunately they can be junk, spam, fake virus alerts like from fake McAfee and such instead of useful information or notifications. You can revoke these sites from sending you notifications as see article link below.

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

They are LARGE' pop-ups that are not letting me use my computer.

If I download a new Firefox will that fix it?

No.

When the pop-up comes up, click and hold the black cross hair and then when over the pop-up window, let go of the mouse key and process explorer will show what process is driving the pop-up. https://learn.microsoft.com/en-us/sysinternals/downloads/process-explorer

You can also use autostart to see what processes are set to run when you login. https://learn.microsoft.com/en-us/sysinternals/downloads/autoruns

Is Mcafee installed? McAfee: how bad is the worst antivirus https://www.youtube.com/watch?v=PkLGvRNVUC0U