What does "have" mean here? and is it okay to say just "I will fire you"?
Unfortunately no, you cannot use "I'll fire you" in this situation. This is because the phrase "I'll have you fired" indicates that the speaker does not have the authority to fire the listener, but he will complain to someone who can fire the listener. The phrase "I'll fire you" means the speaker can fire the listener; that person has a higher rank than the person he is speaking to. "Have" in this sentence means to possess or hold; Paul holds power over Ross. He is telling Ross that he plans to complain to Ross's boss about him dating his daughter so that he will be fired. He can't say "I will fire you" because he is not Ross's boss; he does not have the authority to fire him. However, if he complains to Ross's supervisors, he may be able to have Ross fired. Ex: You better stay away from me or I'll have you fired. Ex: She threatened to have him fired after he harassed her.