Train from Firenze to Roma Termini or straight to FCO

There is only one direct train that will get you there in time, and it gets you there 5 hours before your flight time. So unless you want to hang out at the airport for 4 hours, take one of the connections that will get you there about 1:00pm. The other direct train gets you there at 1:52pm, which I think is cutting it too close. No need for a taxi as you will already be inside the station, so you just change to the Leonardo Express train at Termini. That would be faster (and cheaper) than going outside and getting a taxi and riding in traffic.

By rail you'd have to travel to Roma Termini and then take the Leonardo Express to FCO. There is no direct train to the airport from Florence. There are frequent departures and would easily get you to the airport on time. No need to waste money on a taxi. Alitalia has three reasonably priced 1 stop flights from Florence to Palermo or Catania. This might be a viable alternative and save you €.

It all depends on what time you are planning to leave from Florence.
I only see one train listed direct to FCO. and it leaves
at 7:38 am arriving at 9:52 a little early.
The rest require a connection at Roma Termini

There is no direct train to the airport from Florence.

There are 2 a day. http://www.trenitalia.com/tcom-en

There is only one train you could take that goes directly from Florence to FCO. It leaves at 7:38 AM and arrives at the airport at 9:52 AM. There is a later train, but it arrives too close to your flight departure time. There are numerous other trains you could take that connect in Rome. I would consider taking a leisurely trip from Florence to Rome on the day before the flight, staying overnight in Rome, an then taking the Leonardo Express from the Rome train station to FCO. We have done similar things to what I describe. One time we stayed in Florence and went to Rome foe a night before flying home. Last year we spent a couple of nights in Rome before continuing on to Sicily.

Thank you all very much! For the Leonardo Express, do I need to book that in advance or the day of once I get into Roma Termini?

It is the airport train and I don't think you can buy a ticket on-line. And you don't really need to. The last time we were there they were selling tickets just prior to the boarding. That was a couple of years ago so I would count on it without confirmation. However, I do think you could buy the ticket in Florence.

If you are buying a ticket from Florence, just pay for the Leonardo Express as part of the Florence-Rome ticket. You would actually buy a ticket from Florence (Firenze S.M.N. in Italian) to Fiumicino Aeroporto. That is how they are listed at the Trenitalia website. You could buy well in advance to get a discount for the Florence Rome portion on line several months before your trip, but, like an airplane ticket, it is only good on the train you asked for and cannot be used on any other train as it includes a seat reservation. The Leonardo Express ticket included will have a time window of several hours that you must use it.

You can buy a ticket online for the Leonardo Express, enter Fiumicino Aeroporto as your arrival station. This will save time at Roma Tetmini. Trains leave every half hour or so and take 35 minutes to get to the airport, then you need to walk to your terminal. Remember to validate your LE ticket before you board at Roma Tetmini.

This was great information. Thank you!

Note that the connection offered may be either Roma Termini and the Leonardo Express, or Roma Tiburtina and the commuter rail, either will work.

If you connect at Roma Tiburtina, the station before Termini (all trains from Florence stop in both), you can use the regular Regionale train from Tiburtina to the Fiumicino airport. That solution will cost you less because the Leonardo Express is more expensive, and you will arrive at the airport at the same time or sooner, because the regular Regionale takes a shorter route that bypasses Termini.

You buy the tickets for both legs in Florence, you don't buy the ticket for the second leg in Rome where you change trains.

Another advantage to Tiburtina is that it is a gallery bridge over the tracks with escalators to all the platforms for changing trains and the station lacks Termini's reputation as a den of thieves.

I will be going the opposite way from the OP. Arriving in Rome at 4:20..so planning to go directly to Florence.
Question: just to confirm what I THINK is true:

? I would like to buy BOTH the ticket to Termini (Leonardo Express) and the ticket from Termini to Florence at the airport. Is this possible?

if so...I can go right through to the platform in Termini because I will have the ticket. My understanding is that one must have a ticket before proceeding onto the train platforms.