Train from Florence to Rome airport (FCO)

Booking ahead is cheaper.

Do you plan to fly the same day you travel from Florence to FCO?

If so, that can be a very high risk plan.

There aren't a lot of SMN-FCO direct trains, but that's the easiest route if the times line up for you.

You will definitely want to book the Frecce trains in advance. They're priced like airline tickets and get more expensive as the date approaches.

What time is the flight? If it is after noon, there is a direct train at 7:05 that gets you to Fiumicino Airport by 9:22. The flight is booked, so you might as well buy now to get the best price for a nonrefundable ticket.

But you might look if you can return it at Fiumicino Airport. That would save some logistics. Where are you coming from in Tuscany?

I believe there are only 2 direct trains from Firenze SM Novella to FCO per day
7 am and around noon
Otherwise to Termini then take the LE

I would not do this on a flight day

Have you considered returning car to FCO?
Or dropping along the way-Chiusi then train to Rome for your night before departure?

There are two direct trains per day departing SMN at 7:05 AM arriving at Rome’s Fiumicino airport train station at 9:22 AM. The next one departs at 11:48 AM arriving at the airport at 2:07 PM: https://www.trenitalia.com/en.html.

I would return the car at FCO the day before the flight. I don’t know where in Tuscany you will be but it is likely that you might have to back track north to Florence, since most popular Tuscan destinations are somewhere in the province of Siena, which is already in the way to Rome.
Returning the car at FCO would be much less hassle, besides being cheaper and faster than going back to Florence and taking the train down to Rome.

"They're priced like airline tickets and get more expensive as the date approaches." That is not quite accurate. All train tickets have three levels of pricing. The exception is the Regionale trains that have a single, fixed price. The trains do not follow dynamic prices as is common with US airlines. There are three price levels -- Super economy, Economy, and Base. The first two are discount and come with various restrictions but basically no change, and no refund, The price at each never changes. The base is the standard walk up fare. It can be purchase any time and exchange within 90 minutes of a missed train. A limited numbers of tickets in the other two classes is available for each train schedule. The number available depends on the train. Less popular times will have more tickets available. However, once the tickets are sold there will be no more for that train. So discount tickets for most trains sell quickly and are often gone with within a short period of time. However, sometimes a discount ticket will be available the week before maybe even the day before.

you can find many more trips if you go from Florence to Roma Termini and then take the Leonardo Express to the Rome airport.

However I agree with other posters that it is may be risky to fly out of Rome if you're starting the day so far away. If it were me I'd spend the nite before the flight in Rome.