Fire Extinguisher is an item appearing in The Operative: No One Lives Forever.
Equipping the fire extinguisher reduces the damage sustained by phosphorus rounds.
The item can be obtained in the mission Misfortune in Morocco, Scene 4. It is sitting in a plain sight in the middle of a fenced-off clearing next to a van. Since the clearing is heavily mined, the player will likely require special glasses with a built-in mine detector to safely traverse the field and collect the item. Because these glasses are obtainable only later on in the game, it is necessary either to replay the mission later or navigate the terrain blindly.
Fire extinguisher on Wikipedia