Combination problem. Your defrost is not working for reasons as yet unknown, and to complicate matters, the condensate drain line may be frozen up. If he drain is blocked, defrost water will run into the refrigerator instead of out the drain tube to the pan.
If one of the door gaskets is not closing for whatever reason, the incoming warm air will cause frost in the freezer. In this case, your defrost system cannot defend itself and excess rime forms on the rear cover and fan area. Check the door seals and take the freezer back and floor panels off to acces the area where the coils drain. Type your model number into the lookup on the left to see your fridge in different views.
This is a standard Frigidaire-made refrigerator, so any other topics on this forum pertainng to drain issues will also help you.