Despite the Fire Sages being forced under the rule of the Fire Lord, they still had significant religious power, which included the ownership of the temple.
PrinceZuko visited the temple to learn about the demise of his great grandfather, and how it would help him realize his own destiny. He sneaked into the temple at night and followed a Fire Sage into the Dragonbone Catacombs. While in the chambers, he found and read the Final Testament of Fire Lord Sozin, his great grandfather. Instead, he found out about the demise of his other great grandfather, Avatar Roku, through Iroh.
The temple's architectural layout was not shown very well, although architecturally it seems to be very similar to the temple on Crescent Island. The temple is divided into three sections, and surrounded by a small wall. The hidden entrance to the catacombs is located in the center of the temple, and can only be opened using firebending.
The Dragonbone Catacombs are the underground burial place of Fire Lord Sozin. Its dark, twisting hallways contain many secrets about Fire Nation history. The catacombs received their name because the main hallway is lined with the skulls of dead dragons. The tomb of Fire Lord Sozin is located off this hallway, behind an ornate metal door bearing a figure of the Fire Lord and openable only with firebending. The tomb itself is a round room, lined around the walls with large funerary urns. On either side of the door are statues of dragons rearing back on their hind legs. The room is centered on an enormous coiled stone dragon, which is wrapped protectively around a small urn, presumably containing the ashes of the deceased Fire Lord. In front of this is a wooden table on which are placed the five volumes of Fire Lord Sozin's Last Testament.