https://support.mozilla.org/en-US/kb/push-notifications-firefox

This looks like Web Push notifications in Firefox. Sometimes some sites you interact with may have fake malware or antivirus notice popups as McAfee seems to be the most common mentioned antivirus popups I see here.

What do I do? Everytime I open firefox I get this awful popup virus warning. Do I need to delete firefox and start over?

Hi John and Barbara, as a starting point, you could look at what sites have permission to send notifications and remove the permission. The following article has the steps (this link is to the "How do I revoke Web Push permissions for a specific site?" section):

https://support.mozilla.org/en-US/kb/push-notifications-firefox#w_how-do-i-revoke-web-push-permissions-for-a-specific-site

It is also possible that an add-on is behaving badly. You can review the list of Extensions from the Extensions button to see whether any of them appear suspicious or simply unnecessary, and then Remove them (or use Manage to disable them on the Add-ons page). More information on using the button and its drop-down:

Manage your extensions using the extensions button in the toolbar

Any progress so far?