As noted with prior posts, no doubt flying out of Florence would be "easiest".
In researching that same question for our last trip in April, we decided to instead add in taking the train to Rome for a couple days, visit a few sites we missed last time there, then fly out of FCO via United back to US.
We don't do super early flights anymore (Florence was very early to make connecting Intl flights in Frankfurt or Munich - again United) and I've read too many issues with Air Dolomiti canceling flights and running notoriously late out of Florence (with high likelihood of missing connection). Also, we only travel with carry on luggage, and Air Dolomiti has a much stricter policy for carry on size. None of what we wanted to hassle with at the end of the trip.
That worked out great for us, as weather in Rome was wonderful and we timed evening Mass at St. Peters.
Not sure who your carrier of choice is, United had flights at 9:45 Newark and 10:30 Dulles which would be hard to get to coming from Florence that morning. Of course, your risk tolerance and willingness to fly early, check luggage, run snug schedules, etc will likely provide you with different choices.