Huh and this is on macOS and not Windows?

After a new Release or update for Firefox some antivirus clients can have false posties until they get definitions update though usually this has been on Windows and not so much on Linux or macOS.

It is MacOS 10.13.6.

This thread is the only thread I have seen about this with macOS version of Firefox 115.16.0esr so it may just be a false positive with whatever unnamed scanner you are using. Try checking for updates in this scanner as false positives can be fixed pretty quickly.

James, I thought the problem was solved with your answer because Intego Virus Barrier did not detect the virus any more. But I checked with Intego to be sure and they had me run a diagnosis and the virus DID come in with the Firefox update but Virus Barrier had first quarantined it then eliminated it from ~/Library/Caches/Mozilla/updates/Applications/Firefox/updates/0/. Anyway, the problem is solved for me but not for the reason you gave. Stephen