For security reasons, Mozilla is planning to remove the FTP support from Firefox in 2021. It’s an insecure protocol, and the usage rate is very low at this time (0.05% according to Mozilla). It’s difficult to maintain the very old implementation in Firefox, and there’s no plan to add the support for FTPS.
Starting with Firefox 77, the FTP support will be disabled in the Nightly channel. The protocol will be handled by an external application such as a standalone FTP client or file manager.
Note that Google has also deprecated the FTP support in their Chrome browser. If you’re running a FTP server, it’s time to consider serving files from a Web server over HTTPS, though the concrete deprecation schedule is not yet decided because of the COVID-19 pandemic.
Update: Mozilla plans to make this change with Firefox 82.