Every state has different laws concerning firing. You need to look up the laws for your state. I live in Florida, employers can fire you with no cause. In most situations though, I think that if you told your boss you were leaving and the reason why, they should be sympathetic. Especially if this is not a usual occurance.
* I do not want to scare you and I am not sure what city you are in, but finding a job in Florida is really hard. I have been on unemployment for almost a year. But, I would think that your employer would not fire you for this situation. Go and talk to them, tell them your situation and they SHOULD understand. I wish you the best and I will pray that you do not lose your job.