The Burning Mountains biome has large altitude fluctuations, which are compensated by having slightly more ores. The general theme of this biome is fire, thus the mountains are populated with numerous lava streams, small clusters of Igneous Rock Ore, Lilyfire and new mobs - Fire Golem and Scorching Lens.
Endermen are very rare and mostly spawn inside caves, which partially provides safety from the fire-themed mobs. At this moment, the rare variations can populate the surface with more Lilyfires or add more caves.
Dungeon Puzzle Edit
Most of the islands have a Dungeon Puzzle inside them, usually nearby the surface and with a small cave leading to it. The dungeon consists of special blocks which the player can interact with by throwing Igneous Rock, causing a chain reaction during which the lit blocks will become unlit, unlit blocks will become lit and the walls will propagate the reaction into adjacent blocks.
Once all of the blocks are lit, a Fire Fiend is spawned below. It will make his way to the surface, breaking the dungeon in the process.