You can say the old chestnut, that you were seeking new challenges or the old job had no opportunity for growth.
If it's a big time gap, then you have more problems. If you are a parent, can you say you were spending time with your family? M/F doesn't matter anymore.
If you weren't fired for a criminal act (shorting the till, internal theft), and the interviewer *knows* you were fired, there's the old "personality differences between my supervisor and myself."
All a former employer is supposed to provide is dates of employment, not commentary. Remember, a former employer is certainly not a reference.
Hope this helps a little,
D. D.