In a "serious" fire you should ignore the fire extinguisher get everyone out of the building.

Fire extinguishers are for small fires! If a little oil in your frying pan catches on fire and you don't have a lid readily available to smother it, use a fire extinguisher. But if your smoke alarm wakes you up and you discover your whole kitchen on fire, get out. The fire extinguisher will not help in that situation, and it may cause you to waste time. (Tip: If and only if the fire extinguisher is easily available, carry it with you as you exit. You might need it to use it clear a path to get out.)