Train from Milan to Florence

Since you cannot predict if your plane will land on time, or how long for you to clear immigration, or that your luggage arrives in good order, buy when you get to the Malpensa train station (I assume that is the airport you are arriving at). Trenitalia may have more frequent departures from Milano Centrale, but many swear by Italo Treno. They are usually a few EUR less expensive than Trenitalia, but the timing may not work.

You should wait until you get there,
This for the train from the airport to Milan Central train station:
https://www.malpensaexpress.it/en/
You can use either of these train operators for Milano Centale to Firenze

https://www.italotreno.it/en in
https://www.trenitalia.com/en.html
When you search you need to use the Italian spelling of each city.

Purchase on arrival
I’d just use Trenitalia because they operate Malpensa Express too

You can purchase at kiosk, ticket window, or use Trenitalia easy to use app
Milano Centrale
Firenze SM Novella

Thank you all. I figured that was going to be the response but wanted to make sure.