From zdnet.com
The handling of clicking on FTP links from within Firefox will soon be passed to other applications, as Mozilla will rip out Firefox’s FTP implementation.
A year ago Mozilla announced its intention to shortly disable support for FTP, but it also said it would delay the move pending how the pandemic turned out.
By February, FTP was disabled in Firefox’s nightly channel and it is currently also disabled in the Beta channel. For general release, FTP will be disabled in Firefox 88 released on April 19.