I don't think that's true. Passive mode will also open a different channel as far as I know.

The only difference between active and passive mode is the direction of the data channel. In passive mode, the client will connect to the server, while in active mode the server will connect to the client.

That being said, passive mode may get things working since the firewall may allow connections in the other direction.

Update: I had active and passive reversed. Fixed now. See also http://slacksite.com/other/ftp.html