Fastest train option from MXP to Florence

It does not look like the Malpensa Express train schedule to Milano Centrale is complete; as it only goes up to December 21.
You could take the bus to Milano Centrale, it is running for sure Xmas day
https://www.terravision.eu/airport_transfer/busmalpensa-airport-milan/book-malpensa-airport-milan/

Also you should make your decision about the train when you arrive as the flight is not guaranteed to arrive 8:10am on Christmas day, 12/25/19. Or you should get a ticket that is refundable in case you miss the train because of a flight delay

I don't recall the Malpensa Express running significantly differently during previous holidays. As long as it is operating normally, then those take one hour and depart every half hour to Milano Centrale station (not the line to Milano Cadorna).

You should use www.trenitalia.com.
The Malpensa to Centrale portion is also reported on www.malpensaexpress.it
Make sure you take the Express to Centrale (not Cadorna). I think there is one every half hour, so relax.

Get the Trenitalia smartphone app, tickets after retrieving your luggage: Malpensa Aeroporto to Firenze SM Novella.

Am I not looking on the correct website for the Malpensa to Centrale train station

You must look at the site of the company that runs the trains, not at re-sellers' like ItaliaRail. Even if it is a reputable company, you do not need a middle-man.

The Milano Centrale-Firenze SMN high speed line is served also by www.italotreno.it/en , the private competing company whose tickets are often cheaper.

Unlike Trenitalia, Italotreno has no counters at MXP so you can't buy a through ticket to Florence at the airport station the way you can do when travelling with Trenitalia. On the other hand, there is free-wifi at MXP and Italo has an easy-to-use app. While waiting for luggage, You'll see locals comparing prices and schedules and then buying train tickets on their smartphones.