The Fire Temple was a five-tiered pagoda that sat on a hill near the volcano's base, managed by the a small group of Fire Sages. Avatar Roku once called this temple his home, and it was here he perfected his use of the Avatar State; after his death, the temple was rededicated to him. The ancient temple uses mechanisms that require Firebending to operate. The temple was destroyed during the events of Winter Solstice Part 2: Avatar Roku.
Now, the temple is slowly being rebuilded, to not only honor Avatar Roku, but all the past Fire Nation Avatars