Sulfuron Spire[47, 78] is the fortress of Ragnaros and the entrance to the Firelands, located in the middle of The Throne of Flame at the southern end of Mount Hyjal.
The Inner Spire is presumably inside of it. Unlike other flamegate-entered locations, the Inner Spire appears to be under the spire, and not the flamegate it is entered from.
Artwork of Sulfuron Spire
Artwork of the walls behind Sulfuron Spire
The Inner Spire (accessible in the quest The Firelord)
Sulfuron Spire Raid portal
Cenarius inside Sulfuron Spire