Let's first separate truth from fiction: It is highly unlikely that anyone would know what occurred at your previous employer unless someone just happened to know someone there who could give them inside information. So, that leaves more the ethical question: Do I come clean or not?

No one in this forum can really answer that for you. But, here are a few points to consider.

1. Telling an interviewer a negative experience is not a positive point in your favor. Do not dwell on it, comment on it, try to speak to how unfair it was, etc. Many of us have experienced a firing in our careers, but an interviewer is there to weed you out and if there is any hint of incompetency or fraud, you're out. If you were the interviewer, would you hire yourself after that story?

2. If you feel you must throw yourself at the mercy of your interviewer, describe the situation briefly and show how you learned from the situation and have addressed it in future scenarios. This would be an interesting twist on the behavioral question: "Describe a situation you were in in which your performance was poor." The key with these questions has little to do with the actual event and everything to do with how you reacted to it.

3. We go to work every day concealing our feelings about our job, our co-workers, and our management. We say some things and we are more guarded about other things. Why should our employment history be any different? Information about this event does not need to be volunteered.

4. You might think it is being untruthful to say, "I decided to move on." It leads the interviewer that you left on your terms. Others, may have less of a problem because you DID decide to move on.

5. The interviewer is asking a myriad of questions they all boil down to these:
- Are you competent?
- Are you reliable?
- Will you get along with me and your co-workers?
- Can you communicate?
If you are competent and reliable, can communicate, and can get along with people then give the interviewer no reason to doubt that. You can either do that be risking #2 or by  answering  the  above  questions  with #4.