They come in Firetrucks when 5-10 flames are around. They can be seen in certain missions, such as Rigged to Blow. Up until GTA IV firemen do very little to put out any fires. In GTA V however, they do react to fires and can be called through the emergency services option via the player's cellphone. In GTAIV and GTAV, they will attack the player if he has a wanted level, and Police Officers will never arrest or attack them. In GTA Online, they can become hostile to the Player for no reason, this also happens with Police Officers.