Are there security issues with FTP, or do people just not use it enough because HTTP supplants it? I haven't used FTP in a while to download files.
https://en.wikipedia.org/wiki/File_Transfer_Protocol#Securit... lists many issues. The worst is that FTP transmits user names and passwords in cleartext. Having said that: nowadays it isn’t uncommon to say “FTP server” for a server running SFTP, its secure replacement, so it isn’t clear to me what exactly is being removed from Firefox.
SFTP is SFTP -- a protocol which runs over SSH -- and Firefox doesn't support that. It wouldn't really make sense for it to do so, either; SFTP more or less requires SSH authentication, making it a poor fit for a web browser.
There is FTPS (FTP + SSL/TLS), which Mozilla might support -- but that's rather rare.