Travel to Siena and then to Rome

From Florence take the 131R bus to Siena (1 hour 15 minutes departs from the bus station across from the train station) then take the Flex bus (from the Siena train station) to Rome (3 hours). Enjoy.

@charylm is correct but if you want the Flixbus directly to the Rome FCO airport itself that is often more like a 4 hour bus. There are a few very early and late buses that include a transfer to the airport but if they drop you at Rome Termini you can either use theirs or make to your own way on the train if you want.
https://www.flixbus.com/

What time is your flight? Just a note of caution about trying to get from somewhere 4 hours away to the airport for a flight the same day. The usual advice on this forum would be to spend the last night in Rome versus Siena if you have a flight - especially international - the next day.

Have a great trip,
=Tod

you can't travel from Siena to Rome

This is of. course an exaggeration. but only slightly. The train from Florence to Rome is direct and fast. From Siena you have to change and it is more round about. Bus takes longer. I'd plan to be in Rome the night before your flight (always be in the town of departure for a high stakes flight the night before.

for Siena, I would plan to do it as a day trip from Florence and travel from Florence to Rome either the afternoon before the flight and stay in Rome for dinner and get a cab the next day or come to Rome very late and spend the night at an airport hotel.

It looks like Siena is on the way to Rome but logistically it isn't.

Thanks to all of you for your wonderful assistance; greatly appreciated. Jim

More on Flixbus Siena to FCO:

https://community.ricksteves.com/travel-forum/italy/flixbus-siena-viale-toselli-to-rome