Good question. Can you be fired for having a disease. They answer would be no, you can't, that discrimination. Can you be fired for your behavior, absolutely. Alcoholism is a disease, yet many alcoholics have done horrific things, they have killed people while driving drunk, there are consequences for this. Having alcoholism explains a lot of bad behavior, however it does not excuse it.
It depends. If a person's problems with alcohol spill over to their job, whereby one's functioning on the job us affected; you can be fired for poor job performance. Otherwise, how would an employer know someone is an alcoholic? If one drinks on the job and seems intoxicated, then they should be fired, right?
Without a contract. Yes, you can. You can be fired for your boss not liking the way you comb your hair, for that matter. Unless protected by a contract and/or a union, anything that interferes with your job can result in being fired. What you cannot be fired for is being an alcoholic in recovery. That falls under americans with disabilities act and affords some protections. If you are drinking, you are not protected.