Just to be 100% sure this gets processed correctly.

The blockstate DOES current store the metadata of the block, but I'd
rather stick with using the Block every time to be absolutely certain
that things are handled correctly.

src/main/java/com/gmail/nossr50/listeners/InventoryListener.java

@@ -132,14 +132,15 @@ public class InventoryListener implements Listener {
 
     @EventHandler(priority = EventPriority.HIGHEST, ignoreCancelled = true)
     public void onFurnaceExtractEvent(FurnaceExtractEvent event) {
-        BlockState furnaceState = event.getBlock().getState();
+        Block furnaceBlock = event.getBlock();
+        BlockState furnaceState = furnaceBlock.getState();
 
         if (furnaceState instanceof Furnace) {
             ItemStack result = ((Furnace) furnaceState).getInventory().getResult();
 
-            if (furnaceState.hasMetadata(mcMMO.furnaceMetadataKey) && result != null && ItemUtils.isSmelted(result)) {
+            if (furnaceBlock.hasMetadata(mcMMO.furnaceMetadataKey) && result != null && ItemUtils.isSmelted(result)) {
                 // We can make this assumption because we (should) be the only ones using this exact metadata
-                Player player = plugin.getServer().getPlayer(furnaceState.getMetadata(mcMMO.furnaceMetadataKey).get(0).asString());
+                Player player = plugin.getServer().getPlayer(furnaceBlock.getMetadata(mcMMO.furnaceMetadataKey).get(0).asString());
 
                 SmeltingManager smeltingManager = UserManager.getPlayer(player).getSmeltingManager();