From The TimeSplitters Wiki

The Fire Extinguisher is a nonlethal weapon used to put out fires. In TimeSplitters 2, it appeared on the 1990 Siberia level and the 1972 Atomsmasher level, and it is used to put out fires to open fire doors. To extinguish yourself, aim forwards and walk while firing, or aim toward your feet and fire.It is powered by the CO2 type of ammo. In TS2 bots will try to use the fire extinguish to kill you.