Returning car in Florence

Maybe. It depends.

Some ZTLs are 24 hours. Some are only in effect during some hours (and in my experience - elsewhere in Italy - some are just for show and don't actually take your picture...at least sometimes - but don't count on that!).

Look here:

TRAVELER'S GUIDE TO ZTL DRIVING ZONES IN ITALY

The Limited Traffic Zone (ZTL) in Florence

I had this same question when we had a rental car in Italy last year. We'd gotten our car upon leaving Rome, used it to travel to Orvieto, and during a week in the Tuscan countryside, but we wanted to return it when we arrived in Florence, as we'd be relying on trains for the rest of our trip. In advance of travel, I'd emailed the proprietor of our Florence B&B to ask if we were running any risk of getting a ZTL ticket if we drove into the city to return the car at the rental facility closest to the B&B. He recommended we not try to drive into the city at all. His advise was to return the car at the rental facility at the airport. This was easy to do. The taxi ride from the airport to the historic city center was a fixed 20 Euro, just as the proprietor of our B&B said it would be.

Where are you coming from when you go to Florence?

We have two cars and 11 people and are coming from Venice. I have sort of mapped out the directions to the Hertz office on Via Borgo Ognissanti 137 and assumed it was in a ZTL area. I did communicate with my host but he did not understand my question and thought I was renting a car not returning a car. He did confirm that the office was in a ZTL area. I would not like to drag 11 people to the airport and probably have to get at least three taxis. A taxi from the Hertz office would be much less expensive as our rental is not that far. I plan to ask at the Milan airport the procedure but as of now I think I will call the office when I get to Italy and ask them what to do. Any other thoughts?

My understanding is that you can return a rental car to the Garage Europa on Borgo Ognissanti without entering the ZTL but you must have good directions and follow them precisely or you may stray into the zone. I haven’t been able to find a map that shows how this is done - the garage looks like it is in the middle of the ZTL. I’m returning a car in Florence in September and have been going back and forth about whether to return it to the airport or the more convenient Garage Europa.

We did the opposite and picked up our car in Florence and even in the parts we were driving, it was fairly chaotic, so I wouldn't recommend going in to the ZTL if you can avoid it.

We have returned and picked up rental cars on Borgo Ognisanti with no problems or tickets ever.

Apparently Hertz will notify the authorities and you have two hours for them to issue a waiver. Read on (this is from 2017 but probably still pertinent):
https://www.tripadvisor.com/ShowTopic-g187895-i68-k10294586-Returning_a_rental_car_to_VIA_BORGO_OGNISSANTI-Florence_Tuscany.html

Hertz at Borgo Ognissanti is a small office and its parking should be Garage Europa, Borgo Ognissanti 96. While it lies in the ZTL zone, a few roads are free to drive at all times. So if you manage to arrive from via il Prato - via Curtatone - Lungarno Amerigo Vespucci - via Melegnano - Borgo Ognissanti, you are driving only free access roads and there are no checks. The office will not issue a ZTL waiver as you do not need it.

There are only two ZTL checking points in the zone: the most dangerous is in Lungarno Amerigo Vespucci: if you are coming from via Curtatone you are allowed to turn right on the Amerigo Vespucci bridge or turn left on via Melegnano, but you can't drive straight on the lungarno. If you understand this point, you don't get a fine.

[Please note that in order to reach some Borgo Ognissanti addresses you need to cross this checkpoint and in this case the receiving office should issue a ZTL waiver. But you do not need a waiver for Garage Europa.]

If you are too afraid of ZTL, consider if you can return the car at via del Sansovino. It is in a semi central zone, distant from ZTL, easy to drive from the motorway exit at Firenze Scandicci and in front of a tram stop to the railway station. - You can easily get a fine getting here, if you do not realize that as soon as you see a sign "Firenze" you are in an urban area with a speed limit of 50 km/h and a speed radar just past the sign. Road signals are clear but a lot of people are still in motorway driving mode and get stung.

Thank you to all who responded. I think I have enough information to safely get to the car return. Who thought planning a trip would be this difficult!