Ah, it happens to the best of us. Let me just say that when I was in high school, Waldenbooks HATED me. HATED! Fortunately, I didn't have to put my many high school jobs on my resume (you might be surprised at some of the jobs I did). Anyway, if you have been let go from an employer (and there are so many reasons why this could have happened) and are interviewing for another position, you might feel a little uncomfortable with the prospect of being questioned as to why you left that employer. You know they are going to ask but it's hard to get your head around how to phrase the answer in a way that is honest, but doesn't totally blow any chances you have of landing that next job (or one down the road).
When things aren't going well at work due to some kind of conflict, it's likely that you start looking for another position. So although the timing isn't to your liking, there was mutual recognition that a change needed to happen. And this is how you should think abuot it. Nick Corcodillos wrote an article on how to finesse this conversation in an interview. He helps you work your verbiage so that you are being truthful but also do a good job of explaining both sides of the situation.
Tip from me: practice your body language as well before having this interview conversation!