If you have never interviewed someone for the post of a general manager before, then you shouldn't think that asking a question like this is inappropriate. It is always useful to know whether the individual was fired or has quit and it would possibly benefit you to call his or her previous employers and cross check the details just to make sure that there is no discrepancy. If the candidate quit the previous job to apply for a better one, then an answer like this is acceptable however if the individual was fired for something immoral and inappropriate in the work place then it would be better for you to steer clear of appointing such a person to your firm who could bring some disrepute to it.