Apologies if this has been posted already but I don't see it.
Apparently b-ticket is no longer the official vendor/ticketing site for the Uffizi, Boboli Gardens and Pitti Palace. That has changed to Coopculture, and the ticketing page is here:
https://ticketing2.coopculture.it/catalog/1ADD541B-E62D-9899-7F01-0199CA85E480?lang=en
In addition, if using the Firenzecard reservations are no longer possible online for these particular attractions but must be done by phone. You can still make online reservations for some of the other museums which require them. From the FAQs for the card:
"Reservation is mandatory for the following museums: Uffizi Galleries, Accademia Gallery, Palazzo Vecchio Museum, Brancacci Chapel, Tower of Palazzo Vecchio (Tower of Arnolfo).
Reservation is optional for the Stefano Bardini Museum and the Novecento Museum.
You can make reservation online, through the Firenzecard app or on the website www.firenzecard.it, for Palazzo Vecchio Museum, Museo del Novecento and Stefano Bardini Museum.
For the Uffizi Galleries, the Accademia Gallery, Tower of Palazzo Vecchio (Tower of Arnolfo) and the Brancacci Chapel, please consult the dedicated FAQ"
"For the Uffizi Galleries you need to make reservation calling from Monday to Sunday, from 8:00am to 07:00 pm:
• the phone number 800 615615 (from Italy)
• the phone number +39 055 0354135 (from mobile phone and from abroad)"
See the Firenzecard's FAQ for making the mandatory reservations for others of the museums the pass covers and which require them, such as the Accademia. My 2-cents? Using this card has become more work than it's worth but YMMV.
https://www.firenzecard.it/en/faqs
B-ticket is still the official ticketing site for others of Florence's fine museums, such as the Accademia.
https://www.b-ticket.com/b-ticket/firenzemusei/default_eng.aspx.html
I sure hope Coopculture does a better job managing ticketing for the Uffizi with than they did with the Colosseum in Rome back when they were the vendor for that one! I swear, they changed release dates more often than I change my socks! 🧦