Getting fired is definitely not the end of the world. In fact, I'd say it's more a measure of the employer/employee compatibility. The hardest part of the interview is determining "do I want to count on this guy/gal/team/company every day?" and sometimes it takes getting shown the door to realize that the answer was "no" all along. From my perspective, if you've never been fired, you've never tested the boundaries of your employability.