Fir-apples are not apples

It's a potato. Only the english would name a potatoe for the apple. They are fine taters though. A very old late maincrop variety. It's kept going because of its fine flavour and texture. The tubers can be rather irregular in shape so it is best cooked whole in its skin. If preferred, remove the skin after cooking.
available here: http://www.shropshire-highland-seeds.co.uk/

Its traditional in some parts of Europe to give apples at Xmas packed in fir boughs. Perhaps the etymology of the word lies here.

The eaerliest reference I've sen to them is rather late but here goes:

Frederick slipped down on theother side, and picked up some stones. Then he climbed up again, and tried to hit the thieves on the head with them: but they only said,'It must be near morning, for the wind shakes the fir-apples down.'

It's from the Grimm tale of FREDERICK AND CATHERINE. the reference is clear, but is vexing, fir apples are potatoes... which definitely do not grow on trees. It is certainly an arcane reference that is lost on me.