Storing luggage at train station in Florence ?

If you are staying in a hotel, just email or phone them and ask them if they will store your luggage till you can check in.
Nearly every hotel in Europe will do this for you.
Also, you might be lucky and get into your room a bit early!
I wouldn't bother with the train station, as you'll just have to go back there again to pick up your stuff.

We are staying at a Airbnb. I will ask them. Thank you !

But as a backup plan, the train station does have a baggage check. It's not lockers, it's a manned counter, just like a coat check. Probably 5 Euros per bag? Enjoy your trip!

That is one downside of Airbnb--you often can't leave bags (but certainly no harm in asking.) Check out the website called Nannybag and look for a convenient spot. We used it in Paris and it was very convenient.

According to this page (in English), the left luggage office at Florence train station is near Platform/Binario 16.

https://www.italiarail.com/train-station/santa-maria-novella-train-station

The train station has a baggage check, you exit the main station area and go down the block a bit. Just look for Firenze Deposito Bagagli on Google Maps.There can be a line to check your bags. I checked one last year and it was 6 EURO per bag. When you go to pick them up, make sure you don't wait in the line to check bags (like I did until I asked someone). It's usually quicker when you're picking up.
Agree to see if you can leave your bags at your Air BnB first because yes, you'll have to go back to the station.

Thank you. That is helpful information.