Why does Property Set throw StackOverflow exception?

I know java and would normally put in getter/setter methods. I am interested in doing it in C# with the following code, but it throws a StackOverflow exception. What am I doing wrong?

Calling Code

c.firstName = "a";

Property Code

public String firstName;
{
    get
    {
        return firstName;
    }
    set
    {
        firstName = value;
    }
}