Carbon dioxide is used as a fire extinguisher because carbon dioxide is neither inflammable nor supports burning.
Carbon dioxide is filled in a cylinder under high pressure and when it is released from the fire extinguisher, it comes out all together at the fireplace.
It forms a layer over the burning and cut off the supply of oxygen.
Oxygen is very necessary for the burning process and in absence of oxygen fire can not exist.
Thus, Carbon dioxide is used in fire extinguisher.