Just in case anyone is going to Florence soon and intends to use the card, this warning appears on the Firenze Card website:
"We would like to inform you that from 1st of November 2016 the booking is required to visit the Dome (Cupola). You have to make the reservation at the ticket office in piazza San Giovanni 7/r after the collection of the free ticket."
So in case you missed it, be aware that you can't just rock up to the dome entrance without a reservation.