Train ticket from Florence to Rome

You can buy it today.
I would use Italo because they have a separate priority entrance for their customers to enter the train platforms at Florence Santa Maria Novella train station
https://www.italotreno.it/en

I was in Florence for 5 days last month

Do note that trains to Rome (sold and operated by Italo as recommended above, or by Trenitalia) serve two stations in Rome (and perhaps more). The one you want is probably Roma Termini, which is by far the most central.
And if in doubt, the departure station for high-speed trains in Florence is called "Firenze Santa Maria Novella", often abbreviated to "Firenze SMN".

In addition to the Italo, also check Trenitalia (Italy's largest rail operator) for alternative departure times and prices.

https://www.trenitalia.com/en.html

The station you'll likely be departing from in Florence is Firenze SM Novella. The station you'll likely want to arrive at is Roma Termini, but it can depend on where in the city you're staying.

Editing to add: was posting at the same time as you, Balso, so apologies for the redundancy.

We took an Italo train from Florence on Saturday and there was no separate gate at SMN, just FYI. I actually much prefer Trenitalia myself but the tickets were significantly cheaper on Italo for our date and time.

I actually much prefer Trenitalia myself

What do you dislike about Italo trains, Rachael?

We took an Italo train from Florence on Saturday and there was no separate gate at SMN, just FYI.

@ Rachael you must have not noticed that Italo has a separate priority entrance for their customers to enter the train platforms at Florence Santa Maria Novella train station. I did for sure.

Trenitalia trains were tardy arriving and departing 120% of the time ranging from 15 to 70 minutes. That is why I would avoid taking them whenever one can. I used both Italo and Trenitalia on my last trip a month ago. Aside from Trenitalia being late every time I took them I have not noticed much difference in the service on the high speed trains between Italo and Trenitalia

Trenitalia trains were tardy arriving and departing 120% of the time

How is that possible?

Were Trenitalia high speed trains late 120% of the times? When? Which line? If it happened in summer along the Emilia Romagna line used by drunk teens in the early mornings it isn't a fair description of Trenitalia's service. Same if it happened on the Florence-Rome high speed line, that was opened in 1977 and it is still used by high speed and slow trains.

Italo trains mostly run on high speed dedicated lines where a late regionale can't slow down all the other services. In summer the Cinque Terre rail is a bottleneck that slows down the whole Genoa-Roma line and, not surprisingly, it isn't used by Italotreno. If it was, they would be late just like Trenitalia.

Trenitalia's high speed trains are faster, more spacious and a little less punctual then Italotreno's: 16.4% vs 12.4%.

I know some like to knock the Italian trains for never being on time and claim that the Germans always are. Our experience is that all trains tend to be a little late from time to time and no train system is worse than another. We have always had good luck with the Italian trains being on time.

I have taken lots of Trenitalia trains in 11 trips to Italy, (That's probably about 55 separate train journeys) and only once was a train late arriving.
The train we were on had to stop and evacuate a passenger with a medical emergency.
Otherwise they have all been very prompt, both leaving and arriving.
Can't speak for Italo as I haven't tried their service.

We've used both train lines, and prefer the Italo. But both will get you where you need to go. Just pick the schedule and fare that works best for you.

Italo and Trenitalia are fairly equivalent. An important difference is that Trenitalia runs a more frequent service; so if you have to change plans, it is not difficult to leave one hour earlier or later, provided that your ticket allows a change. If you have to change time with Italo you will discover that you have few choices and more distant in time.

There were only two entrances open to the platforms when I was at SMN on Saturday, and neither was dedicated to Italo--just one for "high speed trains."

I found Trenitalia a bit more comfortable and, this is Covid specific, but they aren't booking at capacity right now so they were much less crowded with more room to spread out. The Italo train I took was completely full. We did have some Trenitalia trains running 5-10 minutes late during the month we were there, but nothing more significant than that.