Best way to get from Florence to Volterra?

The fastest way is to drive. Otherwise there are two ways.
1. BUS
Bus from Florence to Colle Val D’Elsa. Then from Colle you take the bus to Volterra.
2. TRAIN + BUS
Train from Florence to Pontedera (on the Firenze-Pisa line).
At Pontedera, in front of train station you take the bus to Volterra.

I’ve only driven to Volterra, and the way to go by car from Florence is via Colle Val D’Elsa. Considering how curvy the road is between Colle and Volterra, if you have a weak stomach I would probably choose option 2. From Pontedera to Volterra the road goes through the Val d’Era (Era Valley), which is flatter and not very curvy, except for the last few km where the road must climb up the hill to Volterra. Volterra is on top of a hill, so there is no way around that, but the Era valley is easier on the stomach if one suffers from car sickness. I don’t think there would be a lot of difference in time between the two options, and besides the curvy road issue, there is the fact that buses from Pontedera to Volterra are more frequent.

Having recently been to Volterra, not even driving qualifies for fast.

(Having to do a classical concert in the Volterra main piazza, we went to survey it and decide stage placement. So I was there with my colleagues measuring the piazza with tape and drawing plans when an old man comes and asks what we are doing. We could not help doing the same trick as in the old film "Amici miei" and we told him we were surveying for a motorway to cross the town center.)