The Firebeards were one of the seven clans of the Dwarves who lived in Nogrod in the Ered Luin during the First Age. Of the two Dwarf houses of Beleriand, the Firebeards were the least friendly. They fought the Elves over the treasure of Nargothrond, and they also sacked the city of Menegroth which in the end the Dwarves that took part were all killed by Ents. 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 Broadbeams, from the also destroyed city of Belegost, while most went to the halls of Khazad-dum in the Misty Mountains, swelling it's numbers.
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 more prosperous than after the sinking of Beleriand. Also during the Third Age It is likely that they answered the request for aid from Durin's folk in the eventual War of the Dwarves and Orcs.