Ciao!
As luck would have it, my wife and and I arrived in Florence on 11/7/16, the very day that this new requirement went into place. We had to make a reservation for the following day. As it states on the website link below, having a ticket does not guarantee entry to the top of the Dome. You must reserve a day and a time that you wish to visit.
You made need Google translate for the link...if you don't read Italian! :-)