what are fire brigade keys for what do they open?

They may not open anything but serve to operate other functions. Some buildings equipped with elevators have a provision that allows the fire service to operate a key that causes the lifts to descend to the ground floor and then be operated, ONLY, directly from the panel in the car.

There may be other functions, such shutting down the airconditioning or high voltage signs that can be controlled by key or a locked switch.

in G.B. the fire services had a long T shaped key which turned on or off the street accessed connection for their hoses(via the 6x6 ")metal pavement grids----also as below ......old recruiting advert Go to blazes with the London Fire Brigade!! Best wishes to all.