Actually you can't fire for any reason, but for a lot reasons. For example, you generally can't fire someone due to gender, or race, or testifying in court for the public good, etc...
I'd only take a 1099 if I was getting full benefits and equity, or paid accordingly. Although as a contractor on a one month contract I'd probably still consider my side project looking for a job. Whereas as an employee I will be a lot more vested in the company.