ftp://ftp.oreilly.com/pub/freebooks/mh/index.htm is an example of an HTML web page served over FTP with relative links. It's worked fine for 25 years (though I bet 25 years ago the URL was only ftp://ftp.ora.com/pub/freebooks/mh/index.htm, which still works) with very little effort on the part of the redoubtable crew at O'Reilly and Associates. Now it's being broken. Intentionally. For no good reason.
* ftp://ftp.isc.org/isc/ – also available as http://ftp.isc.org/isc/
* ftp://ftp.oreilly.com/pub/ – only available as FTP
For the O’Reilly URI, it’s convenient to be able to click on directory links in the browser and then open HTML and PDF files without requiring another program.
So, it’s nice to have native FTP handling in Firefox for the odd time I’d use it but I can understand why Mozilla decided to remove it.