The firewall should realize that data is still being sent, so the connection is not idle, and not close it. I would double check and run tcpdumps to see which device is closing the connection. Or if you think it is the firewall you could try increasing the idle timeout. Configuration of this differs depending on the version of the firewall you are running.
Fast path sessions should not affect this. It should be enabled as it helps performance.
Thanks, changing the idle timeout proves
anything you commented