Once it clears generally everything is gone and if they clear the history or cache options that basically wipes all traces. If your concern of their usage make their own account to use and then you can have Admin control how they use the accounts. But usually if they can clear cache or history there's nothing you can really do at this point.

Dropa, Then how does the law departments see what a person says if they thought he/she committed some kind of crime?

That is with the ISP they have the history of the tracking. I think this is going beyond Firefox forum help. And you need to seek advise not from Firefox for what your asking.