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).