If I was fired/let go and didn't know the reason, that would likely make me feel very uncomfortable. I have been let go from some of my past jobs, but I was either told a reason that I could understand, or I just knew. I have quit more jobs than I was fired from. As for the ones I was terminated from, some of the reasons were absence (a lot), behavior, supervisor conflicts, no more work to be done, and in one case the owner was clearly mentally unwell and accused me of ruining all of the computer systems (which was totally ridiculous). In the last case, I was just a temp between college years. The temp agency told me that the owner was irrational and that her husband said I did nothing wrong.

If you have been told some reason you don't believe, I'm not quite sure that anyone would tell you the real reason, unless a former work colleague knows and is willing to tell you.