In vanilla Minecraft, these blocks and items can be used as fuel for the furnace. Pixelmon have some inconsistency among its wooden blocks and items and should be fixed to go along with the properties of the base game.
These blocks includes, the Pixelmon version of:
And one more item:
All these blocks and items variance can be used as fuel in the base game while you cannot in Pixelmon. Chairs and couches shouldn't be a problem since vanilla Minecraft doesn't let you used beds as fuel either.