I am getting constant warnings TRojan virus detected constant , under Mozilla firefox heading How do I stop it

Those pop-ups are often caused by a website notification that was accidentally allowed, rather than an actual trojan infection.

In Firefox, go to Settings → Privacy & Security → Permissions → Notifications → Settings. Look for any unfamiliar websites and remove them from the allowed list.

Also, avoid clicking the warnings or downloading anything they suggest. If the pop-ups continue, run a scan with Windows Security to check for anything suspicious.

Do the warnings appear inside Firefox as website notifications, or do they show up even when Firefox is closed?

TROJAN warnings !! constant popups

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 are likely web push notifications from websites you have allowed to send you notifications. Instead of useful information some may send you junk, spam, or fake virus/trojan/antivirus alerts etc. You can revoke/block these sites (mentioned in popups) from sending you notifications as shown in KB link above. These popups are not from Mozilla nor the Firefox web browser.