When I was in Florence in June 2016 I discovered I could extend the time value of the Firenze Card to 5 days. Because the Duomo museums use their own system for entry to their sites, I did the following: (1) I used the Firenze Card to visit all of the sites I wanted to see except the Duomo, Duomo Museum, Campanile, Crypt, Baptistery and dome. (2) During the first 72 hours of my Firenze Card I picked up the Combo Ticket for the Duomo Complex ,which you have to do at the main ticket office across from the Baptistery, BUT I DID NOT ACTIVATE IT. I understand you have about 6 days from the day you pick up your Duomo Combo Card to activate it. (3) On my fourth day in Florence I activated their Combo Ticket by using it to climb the Campanile. Since their Combo Ticket is good for 48 hours after its first use I technically added 2 days of value to the Firenze Card. I used that extra 2 days to visit the Duomo, Duomo Museum and Baptistery. You can use your Firenze Card To jump the lines during that 48 hours because you only have to show the Firenze Card to jump the lines at the Duomo Complex. You then use their Combo Ticket to operate their turnstiles. I did. I was advised I could do this by the person who gave me theCombo Ticket. The Firenze Card sellers either don't know about this or don't volunteer it. To be safe just verify that you have 48 hours from activating their Combo Card when you pick it up at their main ticket office. Hope this helps.
Thank you for sharing this information!
Another little "trick" is to activate your Firenze card around 10-11 am. The card is 72 hours, not 3 days. On the fourth day, you can go to a site like Santa Croce or another site and get in before the 10 am cutoff (time depends on your activation). That way, you can gain admission to another site before your card expires. Think of a major site like Santa Maria Novella or Santa Croce where you can spend 2 hours. Then, use your duomo complex card.