Even if this were true -- which is debatable -- I think the much more likely scenario is that the employer has had problems in the past, and very possibly in the present (whether the fault of the employer and/or employee(s)).

Regardless, I just don't think it would be worth risking highly talented candidates (rightly) misinterpreting bringing up the topic of firing during an interview as an indicator of a toxic environment, and therefore think this is not very sound advice.