Should I make a reservation on the high speed train from Florence to Milan Central (final destination Milan airport).
Yes - if you can commit to a specific departure. If you want flexibility just wait.
Check the difference between lowest ("super economy" (or something like that) and highest price (called "base") right now. Highest price is what it will cost, if you buy on day of departure.
The Florence-Milan route is served also by the private competitor, www.italotreno.it/en
Italo's walk-up fare is called FLEX and not base.
If you decide to go with Italo, you'd better buy online the ticket of the train from Milano Centrale to Malpensa airport, but only when the Italo train will have almost arrived in Milan. Connections between competing companies are not protected.
I assume you are flying home from Malpensa, Milan has 3 airports.