How to get from Florence to Civitavecchia

Not surprisingly There is a train. Good or not depends on what's good for you. For sure it's good enough for millions each year.

From Firenze SM Novella to Civitavecchia on https://www.trenitalia.com/en.html

Use a day of travel before December 8 to see the complete schedules and all the prices. Discounted tickets can't be refunded and changes are restricted. Full price tickets can be easily purchased at the counter up to 15 minutes before the departure time.

Looks like train is best. See https://www.rome2rio.com/map/Florence-Italy/Civitavecchia

Yes - take the train. Note that the journey will require a change in Rome.
Per Dario, to explore sample schedules and to get an idea of the prices involved, try plugging in sample dates between now and Dec. 8th ... the reason being that the full holiday schedule hasn't been published yet for dates beyond then.
Note too that you'll have a choice between the fast Frecciarosa trains versus the much slower (and cheaper) Regionales. You can usually save money on the Frecce's by booking a few weeks in advance.
If you don't want to fool with paper tickets you can explore the Trenitalia App for your smart phone. From there you can conduct searches, book your tickets via credit card, and receive electronic tickets in return as an attachment to a confirmatory email you'll receive after the booking is confirmed.

It's not the holiday schedule, there is no holiday schedule in Italy.
On the second Sundays of June and December all European train systems switch from the Summer to the Winter schedule and from the Winter to the Summer one . It's the notorious seasonal timetable change.

The number of subsidized Regionale and Intercity trains depends on the money allocated by each Region and by the central government. Now the story gets complicated because of budgetary rules, In short the later a poor Region decides, the more trains Trenitalia is able to run.

Believe it or not, most Italians have no idea of these timetable changes when they start looking for the new schedules they find them uploaded and updated.

"It's the notorious seasonal timetable change."
Ah, that explains it. I do learn something new every day on this forum ...