"FTP is an insecure protocol and there are no reasons to prefer it over HTTPS"
There are many reasons to get rid of half-baked FTP features from a browser and leave them to a real FTP client, but that's not the right one.
Sure, FTP is as much insecure as HTTP. So add an S to it to just you did to HTTP and it becomes as secure as well. One reason to prefer it over HTTP is that is much easier to manage remote files with FTP than with HTTP - and WebDAV is not exactly a solution - plus not every machine and his e-dog runs a web server.
But I'm sure Firefox "consumer-oriented developers" meant "downloads" only, what they think everybody does only. Just they should tell it.
PS: there also reason to give some people a FTP access only and not a SSH ones to some machines. The former doesn't give you a shell into a remote machine.