Foxes can spawn in giant spruce taiga, but not in giant tree taiga

Confirmed. I am pretty sure it is intentional for foxes to spawn in neither of those biomes since berries are related to foxes, and those berries are explicitly designed to only spawn in taiga biomes and their variants, and much more rarely in snowy taiga biomes and their variants. This can cause foxes to spawn in giant spruce taiga, which has absolutely no berries. The foxes need to eat!

Note that berries can now spawn in mega taiga biomes and their variants, now that foxes can spawn there.