Inventory Furnace lets your furnaces keep working after you pick them up.

Break a furnace, a blast furnace, or a smoker, and everything inside stays with it. The item you are smelting, the fuel, and anything that has already finished all come with the block. Cooking does not stop. The furnace keeps burning in your inventory, and it keeps burning if you drop it on the ground.
While it is lit, the item looks lit, the same way a placed furnace does. Hover over it to see what it is cooking, what fuel is inside, and what has already finished.
Place it back down and it continues from where it left off. If it was burning, it stays lit, and the same items are still inside.
There are no new blocks and no new machines. Your normal furnaces simply keep their contents and keep cooking while you carry them.

