Comment 31 from Doug T, who was the initial naysayer:

When 92928 lands, I will make this work with the condition that it will be disabled via a pref, and when enabled there is some dialog that asserts the associated risk.

So, yeah, he's not working on it now, but has made some sort of commitment to do it in the future.

92928 is an RFE for the Mozilla networking libs to include the facility to listen on ports, so it would seem to be a legitimate requirement for passive FTP.