keep getting trojan alert mozilla system alert

https://support.mozilla.org/en-US/kb/push-notifications-firefox#w_how-do-i-revoke-the-notifications-permission-for-a-specific-site Are these as small popups on right side of Firefox window?. If so then these may just be 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/antivirus alerts etc. You can revoke or block these sites as shown in KB article link above.