Actually, we have some kind of firehouse - each Storehouse will send a worker with buckets of water to nearby fires if there is stored water. Maybe it's a good idea to extract this mechanic to a dedicated Firehouse building. Thank you for your suggestions and info about Unkown Horizons. I didn't know this game, looks really good.
Since we are making this game in our spare time, the development is not as fast as we would like :/ But I assure you, we have enough motivation to finish it!