"ftp server is IBM mainframe and that's location is in our main office. "
Then why would you be natting inside your own company? So you running what OS z/OS? sftp (ssh) is available on pretty much every OS on the planet..
"those force us to use cmd ftp to get some files"
Not really no.. there are many ftp clients you could run on windows that allow for scripting that support way more features than the built in cmd.. If your using active then install ftp active package for pfsense for your clients behind pfsense..
https://forum.pfsense.org/index.php?topic=89841.0
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