The firehole doors, as well as being open for coal to be put on the fire are used to admit secondary air. Secondary air helps to promote efficient combustion including alleviating excessive black smoke. The firehole doors are partially open a lot of the time. When a round of coal is put on the fire black smoke is produced and allowing air through the firehole door cuts down the smoke, as the coal is burned less smoke is produced and the secondary air can be reduced by slightly closing the doors. When there is no smoke it's time to put another round on the fire.