Train from Florence to Nice

It's about seven hours on the train (11 for a night train but those can be uncomfortable). Have you checked flying?

One way (I think the fastest) to accomplish this is via Thello. I found a fare of 35 euros for the Genoa-Nice segment if traveling on Monday, February 13. For May 30 you could buy a ticket today for just 15 euros, which seems quite the deal. I think there are three trains per weekday on that route.

There's a lot of TrenItalia service between Firenze S. M. Novella and Genova Piazza Principe. That leg requires a connection, and part of it is on regional trains. The cost of the ticket will vary, depending on what kind of train you choose for the fast-train segment and when you buy the ticket. Choose carefully, because the more expensive options can be quite a lot more expensive if bought close to your travel date and yet not save very much time. If you buy the ticket way ahead of time, the cost will be very reasonable, no matter which departure you choose.

Alternatively, I think you could stick with the standard TrenItalia trains to the border town of Ventimiglia and switch there to SNCF. But I think you'd have to buy separate tickets. I believe someone else asked about this just the other day and the answer was that one would exit the Italian train at Ventimiglia and buy the French ticket for Ventimiglia-Nice at the train station.

Your best choice for a same day journey.

FR 9508
Dep 09:00 FIRENZE S.M.N. (Italy)
Arr 10:40 MILANO CENTRALE (Italy)

EC 143
Dep 11:10 MILANO CENTRALE (Italy)
Arr 16:00 NICE VILLE (France)

Thanks to all! Very helpful!