Hi Ken,
Firefighter activity utilizes RFC connection from the GRC system to the plugin system in which FF activity is being performed. You can achieve idle session timeout for the Firefighter by placing idle time restriction on the RFC connection.
As RFC calls are executed thru dialog work processes, so you can set the parameter rdisp/max_wprun_time to a desired value, example 3600 seconds.
Hope it helps. Let me know if you have further questions.
Thanks,
Faraz