FTP can work in two very different modes, based on who initiates the Data connection between the Server and the Client.
1. Active mode (PORT)
If the data connection is initiated by the Server, the FTP connection is active
Both the server and the client will be required to open up ports to listen for incoming traffic.
This can cause problems if you are behind a firewall / NAT router
2. Passive mode (PASV)
If the Client initiates the Data connection the FTP connection is passive.
Only the server is required to open up ports for incoming traffic.
Most FTP servers prefer the Passive FTP connection due to security issues.
All browsers are by default configured to work in passive mode when used as FTP clients.
For full details on how the deploy FTP in both explicit and transparent proxy modes please see the following Knowledge Base article:
Supporting FTP on the ProxySG