They are indeed legal in Florida and are excellent home deffense/shtf tools.
When your rifle is delivered to your ffl you'll be notified, and when you go into pick it up you'll pay the ffl a background check/transfer fee, when the background check is administered this will also register the weapon with the atf, no additional steps required, unless you were to convert it to a short barreled rifle, or add on a silencer.
But ARs in a normal configuration don't require tax stamps or NFA registration.