No one really uses FTP to transmit anything secret or confidential. Most of the time the password to that FTP server is just "guest" and the same file can also be retrieved by HTTP without authentication.
This is tangential to whether support for FTP is still worth keeping in a web browser, mind.
If we're limiting ourselves to HTTP, WebDAV is pretty nice and has builtin support across Windows, Mac, and most Linux distributions.
So...no reason for FTP in the browser, since it already supports HTTP?