Firefox didn't support directly opening and rendering HTML files via ftp:// URLs anyways, it'd just download them like a normal FTP client. I can't even remember the last browser I used that supported that. You can't kill what has been dead for decades already.
Actually, yes, it did, until Firefox 61 when FTP subresources were disallowed from HTTPS, and Firefox 70 when HTML pages over FTP were converted to downloads as well.
