As a former firefighter - do NOT prop open any door (including at the top). Most fire stairs are ventilated/presurized to keep smoke out. Keeping open any door will allow the pressure to drop in the stairwell thereby sucking smoke into it (which you definately do not want)! You can actually have a chimney effect with smoke and fire being sucked into the exit stairwell. KEEP ALL DOORS CLOSED!!
Also, often it is best to just stay in your room (with the door closed) and wait for the firemen to arrive if the corridor is full of smoke or heat.