Back to the US… back to the US… back to the USSR

Apparently, Russia was allowed to substitute the USSR as a permanent member of the UN Security Council back in 1991.

Was that legal?

If so, does that mean that after the disintegration of the United States, that we are currently witnessing, eg Florida could say: ‘Okay, we’ll take the place of the former US as a permanent member of the UNSC?’