We've been in Florence in August a few times - most recently in 2008.
On the most recent August visit, we noticed huge changes from the previous ones - when Florence was essentially shut down. In the olden times, hotels were open, museums were open, and that was about it. We got to know the local Chinese restaurant far, far better than we cared to. There was no entertainment at all. No serious shops were open. There were no Florentines around at all - except for those who worked in the hotels, museums, and few other establishments that ignored local custom.
In 2008, virtually everything was closed on Ferragosto. The rest of the month, most of our favorite restaurants were closed, but enough were open so we could eschew the dreaded Chinese. An astonishing percentage of high-end shops were open - along with almost all of the lesser-lights. It was possible to find a laundry-dry-cleaner, a cobbler, and other such services. There even was some music - if not much. We liked the concerts at the ex-church of Santa Monica, even if we were not thrilled by the concerts at St. Marks English Church. Most importantly, there were even a few Florentines in town.