Firenze Card

Wait until you are in Firenze and buy it there; you won't have to keep track of it between now and then.
Because the card is activated with your first visit, you can certainly buy it ahead of time.

The downside of the card is that should you think you can use it to see ALL the museums, galleries and attractions that it offers, you'll be exhausted in the three days that it is valid.

While Firenze is a great walking city, the biggest advantage to the card may be the free transport between places, especially if your plan is to visit places far from each other. The second big advantage is not having to stand in line at the entrances of all these places . . . except for all the other folks who bought the card.

I book lots of things in advance, but museum passes are something I prefer to buy on the spot. There's no advantage financially to buying it ahead of time.

I am going to buy the card in Florence. I believe you can only get a voucher online. Then, you still have to go to a place that sells Firenze Cards in Florence to redeem the voucher. Also, you could lose the voucher in the meantime. It makes more sense just to buy the Card in Florence rather than online. There is no cost advantage or time-savings online.

Because there are so many sites associated with the Card, I recommend going to the Florence Card online site and prioritizing the places you want to see with it. It operates on a 72 hour system after you first activate it at your first attraction. Note: Duomo - the Cathedral is free and a FC is not needed. But, you do need it for the Duomo museum and to climb the dome. Palazzo Vecchio is open until 9 pm. So, you can see that later in the day and attend other museums during the daytime. Check the days and hours the various attractions are open. Also, pay attention to locations of sights. Then, with pen and paper or electronic thing - organize your time. Have a few alternatives noted. I did this myself. It took a while, but was well worth it.