Trains that leave from Venice

Enter Venezia Mestre as departure station and both italotreno.it/en and trenitalia.com/tcom-en will show trains from Venezia Mestre to Firenze SM Novella.
I doubt there is a single train departing from S. Lucia that doesn't stop at Mestre to let conductors get on.

So that's great news..... but even the high speed trains will stop in Mestre??
This isn't on the schedule and I'm having a hard time buying an advanced ticket - Should we just wait till we are there?

What do you mean with "this isn't on the schedule"? Italotreno runs only high spee trains, so you can't go wrong with them. Trenitalia's high speed trains are called freccia-something.
But you don't need to ask us, just enter Venezia Mestre as departure station, a day of travel before June 9 and you'll see all trains departing from that station.

Ok thanks for the great answers !!!

All trains to and from Venezia S. Lucia also stop at Venezia Mestre, the only rail line onto the Venice islands is via Venezia Mestre.
The inverse is not true, there are many trains which ONLY stop at Venezia Mestre, bypassing Venezia S. Lucia.

I would not say Venezia Mestre is "near" the airport, both require extra transport to the airport.

When are you traveling? You can save a lot by buying tickets for the fast trains 2 or more months in advance, on either Trenitalia or Italo. But if you are going in August, say, the schedule may not have been loaded yet. Also note that with the advance-purchase discount tickets, you are tied to a particular train. If you miss it, you may be out your fare ( for Economy rather than Supereconomy there may be a way to change to a different train, but I haven't checked the current policy)

Here is the Italo website so you can see the times and fares:

https://www.italotreno.it/en

Be sure to use Venezia Mestre as the starting point, and Firenze SML as the destination. In June, for example, they have trains departing Mestre at 12 minutes past the hour at 8:12, 10:12, 11:12, hourly til 14:12, then 16:12 etc.

Trenitalia trains seem to run every hour at 37 minutes past the hour in June.

There is bus from the airport to Mestre station, cost about 8 EUR each. You say "we" so there are at least 2 of you, so a taxi to Mestre should be about 24 EUR or so.

The Economy tickets from Italotreno are 19.90 EUR and can be changed up to 3 minutes before departure with a 20% fee plus the price difference of a full fare ticket, 45.90 EUR.

Trenitalia full fare tickets are 53 EUR.

If it "isn't on the schedule" you may be looking in the wrong place.

If you are looking on Rail Europe - stop doing that.

Look at the providers of the service.