"nicht" is placed in the last possible place in the sentence, here before the infinitive verb, so it negates the verb. As you have two of them it can theoretically negate either one. But normally the modal verb is the one negated, otherwise you would need to pronounce the "nicht" very strongly. So here the sentence means (with the opposite of "can"): "We are unable to hear the fire fighters." If you were to interpret it differently it would mean: "We can overhear the fire fighters." but that is just nitpicking. Nobody would read it like that.
I hope I could be of help.