need to get rid of mozilla system alert and virus alerts, they are fake

Are these as small popups on bottom right corner of Firefox window?. If so then these are likely just 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 these sites from sending you notifications. https://support.mozilla.org/en-US/kb/push-notifications-firefox#w_how-do-i-revoke-the-notifications-permission-for-a-specific-site