Florence Airport

By taxi. The flat fare is €20 plus €1 per bag in the trunk.
The distance from the station is 6km (3.5miles). It takes from 10-15 min to a max of 20-25 with heavy traffic. The airport is tiny and there are very few flights departing from FLR. The check in has never taken me longer than 5 minutes since they opened the new oversized (for that airport) Check-in hall (the old one was cramped). Going through security and walking to the gate is 5 to 10 min max. If you are there 1 hr prior to departure it is enough. However consider that at that time in the morning (8:30) there might be more traffic, therefore plan on 25 min taxi ride just in case.
There is also a bus called Volainbus departing from the station. It costs €6 per person. I have never taken it in the dozens and dozens of times I've departed from Florence.

Most airports have this kind of information on their website: http://aeroporto.firenze.it/en/the-passengers/transport/bus.html The bus station is across the road from the railway station. I'd check the schedule the evening before at the bus station.
For a flight within the EU one hour before departure should be early enough.

Volabus bus stop at SMN station is just outside the train station ticket hall.

I've taken that bus many times.
Just go and look at the schedule on the actual bus stop the day before to check.
The stop, last time I was there, is opposite the main exit of the station, across from where the taxis line up.
Check though, as it may have moved.
You only need to be at the airport an hour ahead, as it is indeed small.

We usually take the bus from the airport to the train station, but every time we fly out of Florence, we take a taxi to the airport.

You will be anxious about catching your flight, it's nice to be picked up at the hotel and dropped off at the correct door to catch your flight home. totally worth the extra Euro.

I suggest getting there 2.5 hours ahead of time. Depending on which airline you are flying, lines to check in could take a bit of time.

2.5 hours ahead of departure is totally ridiculous for an airport like Florence. There are no more than 3 to 5 departures per hour from FLR and all flights are on 80 to 100 seat regional jets, due to the short 5500ft runway. The flight to Munich is a Schengen flight on an Embraer 190, a 100 seater.
I have never ever gotten there more than 75 min prior to departure, and still had more than enough time to spare.

I agree with Roberto.
DO NOT get to that airport 2.5 hours ahead of time!
There is no need, and there is absolutely nothing to do there!
You're only flying to Munich, so one hour is the expected time before your flight.
There are no shops except a tiny duty free, and one coffee place before security and one after, with limited choices.
I was only there last in May.

I took a flight from FLR airport last month (my 3rd time using this airport). Exactly what Roberto said.

Also, I prefer using the bus on arrival, but taxi for the departure. I give it no more than 90-100 minutes total time, from pick-up in central Florence, using a taxi.

Is it hard to take the buses with suitcases? I am sorry, we don't live in the city so we don't have a bus route to compare it to.

You asked about luggage on the bus. It's easy, your bags go underneath in the luggage compartment. If the bus doesn't happen to have this, there is usually a rack on the bus for your bags. If you are doing carry on size, not an issue.

We've taken the bus with 2 larger bike cases, as well as our carry on luggage cases and it wasn't an issue, but a little bit of a hassle.
That's 4 suitcases for 2 people, but we travel with our tandem.

Taxi would be the easiest for you if you have concerns at all.

Unless your hotel is right at the train station where the Volainbus bus also departs from, you should take a taxi from your hotel directly to the airport.

At that time there are no night surcharges, therefore the taxi fare is a flat 20€ plus 1€ per suitcase in the trunk. You don't leave tips to taxi drivers in Italy. So the most you will spend is maybe 22 or 23€.

You said 'we', therefore there are at least 2 of you. The bus is 6€ per person, so for two people double that. But if you are not staying at a hotel where you can walk to the station with your luggage, then you will need to take a taxi from your hotel to the station. I can guarantee you that, no matter how short the distance is, the taxi fare from your hotel to the station plus the bus tickets from the station to the airport, will end up costing you more than taking the taxi from your hotel all the way to the airport (and you'll save yourself some time and the hassle too).

I believe we are staying within walking distance of the train station; but, we can also play it by ear once we get there. It's good to know that both are options. Thanks so much!