getting repeated messages saying system is infected

https://support.mozilla.org/en-US/kb/push-notifications-firefox#w_how-do-i-revoke-web-push-permissions-for-a-specific-site This sounds like you are getting web push notifications from sites you have allowed to send you notifications. Instead of useful information some may send you junk, spam, fake virus/trojan alerts etc. You can revoke the sites from sending you notifications. Alternatively some antivirus clients may still fuss about things like trojans found in the Firefox Cache which is harmless if left alone until overwritten over time or cache deleted.