Seat choice on high speed trains between Florence and Napoli (etc)

Without knowing which train this is, it is impossible to say where these two seats are.
There are lots of different models of trains, made by different manufacturers at different dates with different internal layouts.
You also need to tell us which coach this is, 1st and 2nd class coaches are different, as are sometimes the first and last coach in a train.

However I will say that every train booking website I have come across by default books all seats together if possible.
My guess is that 3B and 4B are facing each other.

There are some Italian train seat diagrams here: https://www.seat61.com/Europe-train-seat-plans.htm#Italy

Note that on this route, the train changes direction in Rome, any seat facing forwards leaving Firenze is facing backwards from Roma to Napoli, and vice-versa.

Here is what I have... does it help to narrow things down?

TRAVEL from Firenze S. M. Novella To Napoli Centrale on 02/07/2019 at 10:13

Departure station
Firenze S. M. Novella
Hours 10:13 - 02/07/2019
Arrival station
Napoli Centrale
Hours 13:10 - 02/07/2019
Train: Frecciarossa 9521
Service: 2° Standard
Coaches: 11
Seats: 3B, 4B

You can choose your seats on trenitalia.com. Select the "choose your seat" box right above the red "continue" button.
The system always finds the closest seats, if not just ask other passengers to switch. Conductors won't care if you are in the right car.

If your seats are apart from one another, politely ask a traveler if they would move so you can sit together. Travelers are pretty friendly and are happy to help.

Going by the "Man in Seat 61" website I linked to above, and assuming it is a model "Frecciarossa 500 (ETR500)" (and not a "Frecciarossa 1000 (ETR400)"), I get to this seating plan: https://www.dropbox.com/s/gx046d6qab0ucm5/Frecciarossa.pdf

Scrolling down to "VETTURE 8-11" (and your post says "coach 11"), then seats 3B and 4B are two aisle seats facing each other.

There are some photos here: https://www.seat61.com/frecciarossa.htm

Hi Gerrilynn :-), when you book your Freccia Rossa train, once you have identified the time and the train you prefer, before continuing dont forget to select the boxchoose sit`. There will be then the possibility to choose the sits you want among the ones available on the map which represents exactly the train you are going to be in, choosing a coach, https://www.lefrecce.it/B2CWeb/seatSelect.do?method=viewSeatSelection&cFID=55TSMk5q10S4
No chance to get it wrong. Have a nice trip.
Anja

Grazie!
It’s a done deal, and I do think we will be seated across from each other. Not the worst scenario 😉 Next time I will have all the knowledge you all have graciously shared.

I do think we will be seated across from each other.

That to me implies you think you will be on opposite sides of the aisle. As far as I can see, you will be facing each other, staring across a small table into each other's eyes, like in a restaurant.

This is my favorite seating plan when traveling with my wife. She hates to ride a train "backwards" and I don't care, so when the train leaves the station, if she is not facing the direction of travel, we swap places. As Chris F stated, this train will switch directions at Termini, so in that case, we switch back again. And with a table between us, a picnic and a bottle of wine, we are all set.