An empty chair placed before the burning altar of God, on Mount Carmel, is an invitation to read and witness the prophet Elijah’s fire challenge given to the priests of Baal (1 Kings 18:21-39). After Elijah rebuilt the neglected altar of God he doused it three times with water. When he prayed to the God of Israel the fire immediately raged and licked up the pooling water. Fire on the chair symbolizes the burning witness you can receive that this account actually occurred. In the Jewish Passover tradition an empty chair is also an invitation for Elijah to return.
To learn more about Elijah and his role in the restoration of the Gospel of Jesus Christ follow this link https://www.lds.org/scriptures/gs/elijah