There was someone else who witnessed the act?
Because I have used my time machine to review the scene, and to me it is very clear that what happened was that:
your boss said something to you.
the way your boss said it to you, you honestly understood that he meant that you had to leave your post for the day. It was difficult to be sure what he meant, it sounded as if he wanted you to have a lunch at KFC.
It was certainly an odd request but you decided to comply to avoid a (rather probable) confrontation if you did not obey him, given his state.
Of course, all of the above is water under the bridge and there is no sense in discussing it again with the new manager; your new manager will be way more interested in the last instructions your boss gave to you before being intoxicated.
If the new manager insists in questioning what happened that day, you explain him it as you remember it (which is how I stated in the above points). If your new manager thinks that missunderstood something and that any correction has to be made, your new manager may explain it to you in a clearer way.
If your boss meant something else, when he gets back to work he can discuss what he did and said while intoxicated with management (if he ever returns, and if he is willing to have yet another talk with management about what happened that day).
Meanwhile, nobody has said anything about firing you1, and nobody has told you to stop coming to work, so everything continues as usual. Should that change, then management would notify you in an appropiate, clear way.
If someone else saw it, it is better to be the one to volunteer the information.
Don't forget the references to KFC, because that may make the boss (and the company) liable for harrasment. State it in a way that makes clear that, if the company backs your boss firing you, it is also backing his behavior (but avoid to look like as if you were making any threat).
Something to the effect that you believe that both (business and you) should just forget and ignore what happened that day, like "it was obvious from his remarks that he was out of his mind/not acting in behalf of the company/etc. and we should ignore all that happened that day, and wish that he gets better." If the official reason for his absence is medical leave, do not mention drunkenness.
1As far as you know, not even your boss -he just asked you to have lunch, do you remember?-