What I hear is: "Hey old lady, you can't pay for our protection so we are going to remove you from your property. It isn't your property anyway, now is it?"

how would that be better with multiple fire agencies, how would anyone even know which one to call when they saw your house on fire?

That is not a difficult task to handle in an open market. If there were no monopolies on doctors, how would you know who to have work on you? That is obviously sarcasm, but this is not a scary problem. People can carry insurance. Many fire stations are volunteer and police... well, I would abolish those clowns and hire private security. There are examples of these working very well in poor areas of Chicago. They do provide service to those that don't pay them. I don't feel like looking them up, but it should be easy to find if you desire.

in the absence of a dominant protection agency with a monopoly on force making and enforcing antitrust laws, how would you prevent the protection agencies from monopolizing and providing terrible service?

Those kind of monopolies are only possible under government regulatory capture. In an open market, there is no possibility of a monopoly.