Generally speaking, when you move to another state you become a legal resident of the new state on the day you begin living in your new permanent home (your "domicile") there.
You can only have one domicile at a time. NJ continues to be your domicile (and your income remains subject to NJ taxation) until you establish a new permanent home ("domicile") in another state. If you haven't yet established FL as your new domicile, you will have to file a 2021 part-year resident NJ tax return.
You can read NJ's detailed rules on residency here: