Like Chris, I have one in each of my cars.
Originally they are called "Mangiafuoco", "Fire Eater", which I think better suits the product. They have been available for many years in Italy, and imported also into other countries like Switzerland, but difficult to find.
http://www.mangiafuoco.biz/index.htmlThey work by sucking off the oxygen in a fire, and having tested it on an open fire (not on a car), it works very well. Important for me, they leave no residue, that corrodes what is underneath, and is difficult to clean off.
You will always find someone to tell you it is not the best solution, you should use Halon, or something similar, very expensive, and difficult to mount in your car.
But I personally think it is a great solution to a serious problem!
Last remark: It should be fitted always in the same place : in the Aurelia I fit it under the glovebox, the holding clips are fitted to the outside of the battery housing. Not visible, but easy to get at, when needed.