There isn't a single museum pass that will allow you to visit ALL museums in Florence, but there are a few options for unlimited visits to some museums for a year. You'll have to decide which museums you would like to visit more than once and maybe at that point invest in that special pass.
Otherwise, just buy tickets each time you go visit a museum, it will be the easiest way to stay flexible and just pay whenever you need to go.
If you don't intend to visit museums more than once, it might just make more sense to pay as you go as you visit museums. A few have "cumulative" tickets so you can concentrate a few days on those that are included and valid for 3-5 days for specific museums, you buy those tickets and focus on those museums for a few days. The Uffizi/Pitti/Boboli have those, so do the Bargello and so does the Duomo, with its monuments.