If you are running an internal ftp server and your traffic passes your pfSense system, chances are it might not work. This was the case for me, and the documentation and forums did little to help. I just simply could not connect at all.
After some experimentation this is what I came up with.
Simply disable the ftp helper on all internal interfaces (on by default in at least 1.2.2-RELEASE and 1.2.3-RELEASE)