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You are mentioning large down/uploads. Since FTP uses both a control and a data connection, the firewall will see two TCP sessions. Not all firewalls will refresh the control connection as long as there is data flowing over the data connection. This means it will time-out after a while.
To prevent this from happening, make sure you configure an idle time-out value that is high enough to incorporate the slowest upload you are expecting for the FTP service.