I'd just like to add to this that ftp is a relatively simple protocol, not particularly complicated to implement. It's not a constantly fluctuating standard that requires a team of 50 developers to keep pace with. Supporting FTP isn't some big technical challenge. The code has been there in the firefox codebase for nearly 20 years now, running just fine. All you need to do to continue to support ftp is nothing at all. If having the small amount of code to do FTP support in there is making your job really hard, that would seem to indicate to me that you're not very good at your job.
I also find the incredibly vague and nonspecific "but security!" scaremongering language to be quite hyperbolic, a repeat of the borderline lies mozilla peddled when they decided to dump xul for webextensions.
It seems to me that the removal of features like this amounts to "I don't use/understand it, therefore I'm going to assume it's not useful to anybody".
Of course, Mozilla being out of touch with its user base is hardly news, so this comes as no surprise at all to me.
(While I'm talking about browsers and particularly mozilla, I'd just like to take a moment to congratulate them on finally getting their market share down below that of edge. They've been working hard at driving firefox into the ground for a long time now, and I'm sure they must be feeling very proud to have finally achieved this important milestone in their seemingly unending quest to achieve that holy grail of 0 users. So I'd just like to say: Nice work, Mozilla!)