I'm married to a police officer so I understand a lot of what you are saying, though I know the last minute calls are less than they probably are for a fireman.
You have to decide what you want.
He does this job. Has he been doing it for a while? Is it that pregnancy has changed your outlook because it is harder for you to do things, and you feel more lonely and isolated, and you are worried about what will happen when the baby is here?
It's an essential job. It is also, like the police, a pretty rubbish job sometimes. It has a big effect on family life.
IMHO you have to decide whether a) you love him and want to be with him, in which case you have to accept that this is his job and this is the effect it has on your life, or b) you really can't take it anymore, in which case there isn't much option but to either leave him or insist he leaves his job, with all the knock on effects that has.
If you decide you want to be with him, you have to accept it. It's fine to feel down about it, we all do and I know I certainly struggled with a young baby and DH doing shift work. But you can't take it out on him because he can't change the nature of his job, bar leaving it. Taking it out on him just increases the resentment for everyone.