Passive & active modes are selected by the ftp client, and if the server supports the mode it will allow the requested connection to be made.
Having said that whether or not passive ftp can be used may well depend on which version of the firmware you are using - it was not properly implemented in fw versions up to 1.07, this is supposedly fixed in the 1.08 beta, however I have not personally tested it and cannot say if that is indeed so.
Be warned - setting up a passive ftp server behind a NAT router is not as simple as forwarding port 21, it will require a range of ports to be specified at the ftp server and the same range forwarded at the router.