The Firebeards were one of the seven houses of the Dwarves who lived in Nogrod in the Ered Luin during the First Age. The Firebeards were most renowned for having the greatest crafters and smiths, even by Dwarf standards in Middle-earth. They were also great masons, for they aided the Noldor build Nargothrond.
Of the two Dwarf houses of Beleriand, the Firebeards were the least friendly. They fought the Elves over the treasure of Nargothrond. Their lust for jewels and other treasures led them to the murdering of King Thingol of the Nauglamir, and the Sack of Doriath. After this act of treachery as they were returning home they were waylaid by Beren Erchamion and Elves of Ossiriand in the Battle of Sarn Athrad. Those few that survived fled into the woods and the arms of the Ents who massacred the remainder.
They did however, help the Elves and Men in the Wars of Beleriand.
In the War of Wrath their great city of Nogrod, was destroyed and sunk into the sea along with much of the Ered Luin. Some Firebeards stayed in the Ered Luin to help rebuild what they could with the remaining Broadbeams, from the also destroyed city of Belegost, while most went to the halls of Khazad-dum in the Misty Mountains, swelling its numbers and bringing much craft and lore with them.
After thousands of years it seems the Firebeards may have parted ways with Durin's folk after the awakening of Durin's Bane, and returned to the Ered Luin, where things were likely much better than after the sinking of Beleriand.