To New York City firefighters, who often put in 24-hour shifts, the firehouse can be a home away from home between emergency calls. It is where they cook meals, conduct training, lift weights and trade notes on the latest internal policy or city political imbroglio.So when the fire commissioner issued an order last month forbidding firefighters to have any “material presenting opinions or viewpoints” in their engine or ladder company quarters, it struck a nerve with the rank-and-file and elicited outrage by officials of the department’s largest union, the Uniformed Firefighters Association.