To answer your question, it depends on the state. In both Oregon and Washington states (where I've lived and worked), you're an "at will" employee which means that you can leave whenever you want and your boss can fire you whenever s/he wants.

Since you were fired, be sure to check into unemployment benefits even if you are only planning on being unemployed for a short while. Money is good, yes. Also, if you have medical benefits, check just when they'll run out.

*hugs*