The rental car return for several companies (maybe all) is no longer at the Interpark Garage Europa on Borgo Ognissanti.
Now it appears that you may be asked to return the car at the Firenze SMN underground parking station, where the Sixt rental return is also located..
These are the directions from Piazza Taddeo Gaddi to the SMN underground parking
Piazza Taddeo Gaddi is a good starting point for those coming from south of Florence (Siena province or Chianti) using the A1 FIRENZE SCANDICCI exit. Therefore set your navigator to go to Piazza Gaddi first, then use the above Google Maps directions.
If you are picking up at the same Firenze SMN underground garage (Sixt is there for sure), these are the reverse directions from the underground garage TO Piazza Taddeo Gaddi.
I suggest you use the Street View feature in Google if you want to see what the actual route looks like in pictures.
From the freeway exit (FIRENZE SCANDICCI) to Piazza Gaddi, or viceversa, you don't have to worry about anything (no ZTL) except make sure you don't travel on top of the lightrail tramway tracks (that would be something only Mr. Magoo would do), and make sure you stay under the speed limit of 50 km/h, because there is the infamous Speed Camera of Viale Etruria which reaches a record of about 1000 violations recorded every single day. The speed camera of Viale Etruria is located here approximately near the IP gasoline station, it is overhead and records speeds for about 150 feet in each direction. So pay attention.