Firetree tribe

The Firetree tribe (or firetree trolls ) was one of the forest troll tribes that joined the Horde during the Second War after the Horde rescued their leader Zul'jin from the human forces. The Second War ended with the defeat of the Horde, but Rend Blackhand refused to give up the fight, some orcs, trolls, and ogres followed him, including the Firetree tribe, and they have become known to others as the Dark Horde.

History
Commanded by Rend Blackhand, the Dark Horde has not given up the battle to claim Azeroth, they are essentially still fighting the Second War, even though Rend's forces are significantly smaller than Thrall's Horde. They know that they stand little chance of success, despite their alliance with the black dragonflight. A group of warlocks drives the Dark Horde, which has not relinquished the original Horde's involvement in demonic magic.

The firetree trolls revere the memory of Zul'jin and consider the Smolderthorn tribe acceptable allies, but despise all other forest trolls as deserters, particularly the Revantusk tribe, which is loosely affiliated with Thrall's Horde. Ironically the Revantusks consider the Firetree trolls traitors for failing to leave Blackrock Spire and aid Thrall and his followers, for the Revantusks believe that Thrall leads the true Horde.

In general, trolls outside the Dark Horde have long since decided that the Firetree and Smolderthorn trolls are insane for allying themselves with the black dragonflight and continuing to fight an unwinnable war (if, indeed, it can be called a war).

In World of Warcraft
Strangely they are missing in World of Warcraft when they are supposed to be in Blackrock Spire. They may however be added in the new Blackrock instances/raids when Cataclysm is released.