The forbidden zone in Florence is roughly what was enclosed within the former city walls. You can still see the walls at Porta Romana, along viale Petrarca and viale Aleardi; there are no more walls but alleys circling the inner core of the city continuing via del Ponte Sospeso - Ponte alla Vittoria - viale fratelli Rosselli - Fortezza and viale Filippo Strozzi - viale Lavagnini - viale Matteotti - viale Gramsci - viale della Giovane Italia - Ponte S. Niccolo - and the the viale up to the hill going to Porta Romana again.
You can assume you cannot enter the city core. Actually, there are a few allowed itineraries crossing part of the core, but you should know what you are doing and these itineraries may change with time. (For example, access to railway station through via Nazionale will be no more allowed after April 1st). What does not change is the circle around the city along the former walls.
Please note that you can get fines also out of the city walls circle. There are radars for speeding and there are a few streets that have a direction reserved to buses and taxis. If you dodge these rules you can get a fine everywhere and at any time. The rule that in built up areas the top speed is 50 km/h is severely enforced and much more fines are issued for this form of overspeeding than for ZTL. As soon as you see a sign that you are entering a town or a city, you should expect a speed radar monitoring if you are really slowing down.