where would you collect/drop off hire car in florence

No worries, Gerard. First of all which company are you using for your car? In any case, they are nearly all in the same area, around the intersection of Borgo Ognissanti and via Maso Finiguerra (in either street). You pick it up from there and return it there (unless you return it somewhere else).
The airport is another option, however it will generally cost you significantly more. The ZTL (Zona Traffico Limitato) is basically anywhere East of the intersection between the two streets above. All rental companies are west of that intersection, i.e. outside the ZTL (via M. Finiguerra is also out of the ZTL) So when you pick it up, go west, when you return, there is only one way to do it: through via di Melegnano. That's the only way to access that intersection that doesn't get you into the ZTL. When you get your car, the company (I usually use AVIS, on Borgo Ognissanti) will give you the map with the highlighted itinerary to get out and get in again. Just post where you are going after you pick up the car, and where you come from when you return it, and I'll give you the exact itinerary.

I returned my car at the airport. There is an express bus from there that will take you to the train station for 5€.

Roberto, many thanks, I have been looking at sixt or europecar.Google street view shows Avis, and others all on the same via.
We are getting a car, knowing all the insurance issues,three days hire in and out of florence, staying just out of Siena as our base. Regards

I cut and paste the directions to AVIS I gave the other day on another post. All car rental offices are near one another. Sixt is exactly in front of AVIS. Using Google Maps - Get Directions, enter the following: A: Siena, Italy (or enter your exact departing point near siena) B: Via di Melegnano, Florence, Italy
click on "add destination" C: Borgo Ognissanti,128, Florence, Italy Make sure you enter ,128, after Borgo Ognissanti, otherwise Google sends you all over the place (including the ztl). The resulting itinerary is the only one you must take. Anything east of that blue line plotted on the map by google is ZTL. If you follow the instructions of that itinerary you will be ok. The point C on the map is in front of the AVIS office. The big indoor Garage Park where you return the car is 15 feet past the AVIS office (which is not actually at no. 128, but sometimes Google is totally off on numbers). Note: At the end of the Raccordo from Siena, there is a big roundabout with a funny diagonal column sculpture. Follow the BLUE signs pointing to FIRENZE on SS2 (The green signs put you on the A1 freeway). immediately after Galluzzo, you'll see several gas stations on the left side (Agip, total erg, Tamoil, etc.). Refuel there before returning car. The last gas station is probably a little ahead when you have to turn on via del Gelsomino (I think it's a Q8 gas station). When you arrive in front of Porta Romana (City Gate) (there is roundabout with sculpture with woman with piece of marble on head), make sure you take the boulevard with trees along the ancient walls. Don't go through the Porta Romana gate (ZTL). The ancient walls will be on your right, you'll drive along them for a bit. Don't go through them or you get zapped by the ZTL cameras. Follow the Google.

Roberto thanks for the great description. Any reason you prefer Avis over the others?
Regards

No preference. I use whichever company gives the best deal at the time of my travel. But if the price is similar I prefer a company affiliated with an American company, rather than a local company because it makes it easier to dispute a charge when I am back home, if I need to. For some reason Avis or budget (same company) happened to give me the best price when I tried. But if it had been National, or Hertz, or Dollar (all of them in the same street or on via Finiguerra) I would rent from them too. Said this, I don't trust any rental car companies, particularly in Italy. They always try to slap you with some extra fee for whatever reason. Avis in Florence charged me for gas (petrol) even when I returned it full. Sicily by Car never reimbursed me for a burned headlamp I had to replace for them, budget charged me $300 for dog hair cleaning fee (and that was in the US). No matter what, you always have to watch your back with those people. Therefore review your bill carefully when you return your car, because chances are they'll try to pull a fast one on you.

I don't see all this fear about the ZTL. The entrances where the overhead cameras are located have a big bright red light when the ZtL is in effect. If you happen to see a red bright pointing at you, just don't go through and turn around. Plus if one follows my directions I gave above, you'll stay out of it.
The ZTL is also marked by the Restricted vehicular Traffic Sign (a white circle with a thick red border, see below link for common EU signs). http://en.wikipedia.org/wiki/Road_signs_in_Italy PS: via del Sansovino is not north of the station, is on the way to the Isolotto district on the west just across the river.

Great information from Roberto. For me, it still looks a bit skinny getting into and out of this area. Here's a link to a map and good article about the ZTLs. http://www.florence-on-line.com/maps-of-florence/florence-driving-map-ztl.html. We're planning a trip now for my wife and her daughter. They will be going from Florence to Siena where they will rent a car. For us, we're just too worried about the ZTLs to rent in Florence.

We picked up our car rental arranged through AutoEurope.com at Hertz's location at 53 Via del Sansovino Florence. It's approx. 2 1/2 miles north of the train station and can be reached by cab. They can give you good directions on how to get out of town staying far away from the ZTL. We turned in our car in Orvieto, an hour north of Rome, with no drop off fee. Hertz's office is across from the train station. A rental car in Italy is very enjoyable, and we didn't find navigation to be any problemeven without a road map or GPS. We enjoy rambling from hill town to hill town.

Thanks to all. Roberto's directions were spot on, I used Google maps and even on nexus 7 was easy to follow. I later, looked at directions to the autostrada a1, which takes you north, obviously looping away from the zone. Again easy to follow. It would be helpful if a map layer of the zones, not just in Florence were available.
Regards

Note: If you are going into Venice by auto on the causeway, they have automatic radar cameras catching speeders. I found out the hard way. Otherwise, driving in Italy is no problem for someone that's a good U.S. driver.