Firestarters can perform their intended animation on just about any block except when looking at completed pit kilns: https://i.imgur.com/gak6BL8.png When you right click them, the player just swings the firestarter in the direction of the pit. I eventually figured out that you can light it by holding shift, but I'm not sure if holding shift as a requirement is intended for pit kilns when it's not required for anything else