Firing for any reason? I doubt it. Someone posted, that you can be fired for the clothes you wear, etc. That would be called discrimination, which is against the law in any state. As far as time or money, to fight for your job? Any lawyer would pick a case up like this in a heartbeat. They make money off the win, just like you would.
Anyhow, back to the initial question.
#1 - Unless you do something grossly incompetent, like: disregard for pt safety, giving meds without an order, or errors of ommision and commision, etc. The immediate supervisor rarely has the power ti fire or hire.
In our hospital, which is joined with one of the countries #1 hospitals, you can only be fired by your director, and usually with a blessing from Human Resources.
#2 - It does not make any sense for your supervisor to say, whet he(she) did. Currently it is much harder to get part time positions filled then full time. Everybody wants full time, those positions go in a heart beat.
#3 - All hospitals have policies, regarding these situations. Check out your Human Resources dept., as well as your hospital policy manuel. Do it now, don't wait until the situation occurs. This essentially falls under the catagory, of blackmail, which is also against the law.
#4 - Get proof of the alleged statement by your supervisor. Make sure you can back your statement, so that it is nopt your word against theirs.
Last - six months of experience at full time, is significant. Don't pay any attention to the lines your supervisor feeds you.
Any other questions, post them.