Under Federal laws/regulations, a person that is active duty Military is considered a resident of whatever state they have orders for.
So, a non-resident of FL that is active duty Military and has orders for FL, is considered a resident of FL.
FL state laws allows non-prohibited residents to transfer firearms amongst themselves without needing to use a FFL dealer.
Therefore...
It is legal under Federal and FL laws, for you (FL resident) to sell a firearm to a person that is active duty Military with orders for FL.
27 CFR 478.11
State of residence.
The State in which an individual resides. An individual resides in a State if he or she is present in a State with the intention of making a home in that State. If an individual is on active duty as a member of the Armed Forces, the individual's State of residence is the State in which his or her permanent duty station is located, as stated in 18 U.S.C. 921(b). The following are examples that illustrate this definition:
Example 1. A maintains a home in State X. A travels to State Y on a hunting, fishing, business, or other type of trip. A does not become a resident of State Y by reason of such trip.
Example 2. A maintains a home in State X and a home in State Y. A resides in State X except for weekends or the summer months of the year and in State Y for the weekends or the summer months of the year. During the time that A actually resides in State X, A is a resident of State X, and during the time that A actually resides in State Y, A is a resident of State Y.
Example 3. A, an alien, travels to the United States on a three-week vacation to State X. A does not have a state of residence in State X because A does not have the intention of making a home in State X while on vacation. This is true regardless of the length of the vacation.
Example 4. A, an alien, travels to the United States to work for three years in State X. A rents a home in State X, moves his personal possessions into the home, and his family resides with him in the home. A intends to reside in State X during the 3-year period of his employment. A is a resident of State X.
