Do you still see this file in the Downloads Manager (Library)?

See also:

• https://support.mozilla.org/kb/i-found-fake-firefox-update

If you get a pop-up message asking to update Firefox or plugins or scanning for malware then such a message is likely a scam and you should never respond to such an alert to avoid getting infected with malware.

• Only update Firefox via "Help > About" or by downloading and installing Firefox from the Mozilla server and never via a pop-up or link on a web page.
• plugins should only be updated via the plugin itself or by visiting the home page of the plugin.

If you only saved the firefxo-patch.js file to disk and not tried to run it then you can just delete it.