You need a Harry Potter solution to protect the Philosopher's Stone only from bad people! Maybe a riddle involving hundreds of fake keys with the real one in there somewhere? Or a system that requires two or three people to be present to activate a mechanism that gives access to the key.
Anyway, essentially you have to allow the fire department's key obtain access to your building, but not allow anyone in the fire department to use the key when they're not supposed to, nor allow anyone who has a black market copy to use it.
I like the alarm idea. When the box is opened, in triggers the security alarm.
How about this: make a tube that connects to the top of the box and goes up inside the building. At the top of the tube is a set of real keys held in place by an electromagnet. When the fire alarm is activated, it releases the magnet and drops the keys down. The automatic fire doors work the same way.
You could also have an intercom system at the box, where someone could convince you to activate a remote control that drops the keys down in to the box. Or you could have a code pad that releases the keys, and make the fire department call you to get the code from you.