Policemen and firemen went to their profession with some reason - money, thrill, whatever. And once they're on the crime scene, they know that their uniform obliges them to act because this is what civil people expect. In their case, if they save a person, it's good for their career, if they don't want to do it but still help someone (which would mean that it didn't make them happy but they still did a selfless deed) in was because their sense of duty, because otherwise they know it will be hell (if they just leave someone die there) - either they will personally feel bad, or people will judge them. Thus, they still do it for selfish reasons.
Sorry, I just cannot think of one single good reason why would you die for someone that you know absolutely nothing about. It doesn't make sense, like if you would be asked now to kill yourself so a boy in Andorra will live. If you don't know anything about him, it won't convince you to do it just for the sake of doing it.