The Fire Mountain is a large volcano that appears in The Land Before Time XIV: Journey of the Brave. It has a similar appearance to the Mountains that Burn in the first movie, though, judging by the different surroundings of the two mountains, they aren't the same.
The Fire Mountain is where Bron is trapped throughout the film. Littlefoot, Cera, Ducky, Petrie, Spike, and Etta go there to help rescue Bron. It is where Bron and his herd had traveled to on the way to visit the Great Valley. The volcano had erupted, but Bron made sure everyone got to safety before him, including the clumsy Wild Arms. It is also responsible for singeing Etta's wing.