Actually, most dry powder extinguishers use ammonium sulphate as their primary ingredient. I've been around this powder once (used to put out an engine fire in a car I was driving at the time), don't recall any laxative properties.
Baking soda is the best for kitchen fires, though it's sometimes hard to apply where it's needed. Dry chemical seems bad for kitchens, to me, since you'll inevitably contaminate the whole kitchen with the powder (which, while not directly toxic, doesn't taste at all good), but it may be the only choice in pressurized extinguishers since CO2 tends to blow burning liquids around (potentially spreading the fire if you don't kill it instantly) and Halon types have been banned for years.
Do *not* try to use flour to put out a fire -- if you disperse it into the air, it can ignite and *explode*, doing a lot more damage than would be the case if you simply dial 911 and wait for the fire department to arrive. With electric ovens, it's often sufficient to simply turn off the oven, leave the door closed, and wait -- the oven has little air circulation and removing the heat source will often kill the fire in a couple minutes anyway. Gas ovens, unfortunately, don't permit this trick, since they have ventilation for the gas flame.
For fires on the stove top (I occasionally get a flare from a hot burner when I flip onions I'm sauteing or similar), a pan lid is perfect -- just cover the burner or pot, and wait, the fire will go out quickly from lack of oxygen, and you can often avoid major damage to the food.