The issue isn't that FTP is all of a sudden an important protocol. It has been chugging away in the background without much fanfare for decades. The issue with Firefox removing it largely comes down to convenience: it's been included in browsers for so long that it's now the primary means (in some industries) for non-technical users to access files.

This whole ordeal kind of reminds me of IE8. Whether good or bad, companies stuck to what they knew and what tools they used to carry out their day-to-day. I can easily see updates being avoided to keep FTP functionality at the expense of newer security issues being patched.