I'm going to second most of what sparksals had to say.
My DH is also in law enforcement, although I cannot get more specific about it than that about which branch, etc. Now, I did know that going into our marriage so I had time to come to terms with it.
What I think most is that right now you are fearing the unknown (understandable) and you're probably replaying the horrific scenes from "Rescue Me" and "Backdraft" in your head right?
As sparksals has said, I would not put my foot down--just yet anyway. I would hope that you and your DH could have a really honest discussion--one that unfortunately includes the more morbid aspects of his job. Such as, are you financially ready to be by yourself in the event something happens? Are you and the children (if you have any) provided for? Frankly, you should BOTH be prepared for this. Is there a will in place? What will happen if one of you becomes disabled or injured on the job?
Then I would ask him to find out if there is a local support group for spouses of firefighters, etc. Talk to some of those ladies and ask them what they think. Find out how they cope. And it wouldn't hurt to have this discussion with a marital therapist too who can help guide the situation and point out what is rational and what is not....not to mention, keep things on target without the conversation becoming overly heated and/or upsetting.
I worry about DH every time he leaves on assignment. While I recognize he has a very high level of training and knows what he is doing, that doesn't stop me from worrying. It is something I deal with every day. And even though his job is dangerous, DH is always telling me that it is just as likely that I too could end up seriously injured or worse just by doing normal every day things...at least he is trained to cope with the unexpected while I, am not.