MFIRE is a mine ventilation and fire interaction computer simulation program. It allows transient state ventilation simulation and features mass based flow rates, natural ventilation, air recirculation, fan curve fitting, air-flow reversal and product of combustion tracking. MFIRE was designed to accommodate the needs of the mining industry for routine ventilation calculations and mine emergency planning. This paper describes recent work by the US Bureau of Mines and Michigan Technological University to improve MFIRE by incorporating humidity in the mass flow and heat exchange calculations, a data file preparation program, and variable reference density in the network calculations. Recent b bug fixes are also discussed.