Just now, I was able to use trenitalia.com to purchase three tickets from Florence to Rome on June 29th, and it looked like there were many other dates available to purchase, as well.
Once the bookings are open, tickets can be purchased up to 180 days in advance: September 27 starting today..
Did they upload also the Regionale and Intercity trains?
Trenitalia always uploads first the high speed trains with reserved seats and discounted tickets. The Milan-Rome high speed trains are always the first to be published, but what about the Freccias from Rome to Venice? Those trains call at Florence, too.
I reserved tickets for the Frecciarossa 9591 from Florence to Rome.
Yes, FR9591 starts its run in Milan. They have uploaded the backbone, I wonder about the secondary line from Venice to Rome and Naples.