picking up a rental car in Florence

Review your Florence itinerary, and decide if you actually need the car when you first arrive. Tourist Florence is pretty walkable. If you're picking up at the beginning of your visit because of day trips during your stay, you'll need a safe area to park, and you'll need to avoid driving in the ZTL areas. If you're picking up at the beginning because your lodging is outside the city, you have a couple of choices. For airport pickup you can ride a bus from downtown out to the airport. The airport has easy access to the highway, although you'll pay an extra charge for pickup there. You could also walk to a number of rental offices on Via Borgo Ognissanti just south of the ZTL, though carrying luggage would be a pain. There are a few other locations in-between these.

Reserve through Autoeurope and pick an office location on the Borgo Ognissanti. Walk there and pick up your car. Be sure they show you the way out of Florence that does not take you through the Limited Traffic Zones (ZTL) which lie on both sides and do not permit non-resident driving. There is a wealth of information about this topic on this site and on SlowTrav.com, but basically you want to drive straight west and then get out of town.

I agree with Doug on the Autoeurope location. Ask them to highlight a map with the route out of town that avoids the zones. And it's only a few minutes walk to the train station.