As various airtanker and helicopters companies under contract with State or Federal firefighting agencies two important things must happen, The first is that aircraft undergoes maintenance to be sure that each aircraft in the companies fleet meets various safety and maintenance requirements. All features in the aircraft are looked at including but not limited to engines, pilot controls. the airframe, etc. See this January 30th Facebook post from Dauntless for more information and pictures about their winter maintenance program.
In addition to maintenance, annual winter/spring training of pilots and crews happens, including training for new pilots and refresher training for pilots who have previously flown fires for the company. The specifics of new and refresher training is usually mandated by the agency who contracts for the aircraft, covering the specifics of flying fires, aircraft safety and the like. See this short February 19th video from Aeroflite Aerial Firefighting on Facebook for a short glimpse into this training.

