Furnace
A fire door is a door that leads to something full of fire. That thing is a furnace.
ろ
いろり
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You're using the furnace to melt tar to put on the road (ろ).
An update is needed before you can continue but you have more than one WaniKani tab open.
Finish up what ever you are doing on the other tab(s), then close them to continue.
The update will complete automatically once the other tab(s) are closed.