The Mountains That Burn are not as steep as the Smoking Mountains. They constantly dribble vast rivers of lava. The rivers of lava somehow create tar which forms sticky pits. The volcanoes booming and hissing provides a constant background. There is also red-hot molten rock raining down. Few plants grow here. There is quite a lot of crumbly rock around the lava.
When Littlefoot's mother is explaining to him how to get to the Great Valley, she tells him that it's "Past the rock that looks like a Longneck, and past the Mountains That Burn." Later, when Littlefoot looks into his tree star after his mother dies, her spirit asks him if he remembers the way to the Great Valley, and when he shakes his head that he doesn't, she tells him again.
As Littlefoot and his friends are getting close to the Mountains That Burn, ash starts to fall from the sky. When Cera decides to break away from Littlefoot's group with Ducky, Petrie and Spike, her "route" to the Great Valley takes her and the others through them. Spike stops to try to pull a plant from the ground and Ducky stays back to coax him on, while Petrie falls from Cera's back as she jumps a tar pit. Cera doesn't notice either and keeps walking.
When Littlefoot goes to help his friends he finds Ducky and Spike on a rock floating in a lava river. He pushes another rock in and uses it as a platform to get Ducky and Spike off the rock they are standing on. The three jump to solid ground with the help of a third rock floating down the river. Then they find Petrie struggling to free himself, and they jump into the tar to pull him out (with Littlefoot holding on to a stalagmite in the tar with his tail), but end up trapping themselves in the process. Cera, meanwhile, continues to travel but ends up in the territory of some hostile domeheads. When they corner her, the other children appear and scare the lizard-heads away, appearing to be a monster, as they are covered in tar from the tarpit they escaped.
Petrie mentions the Mountans That Burn in the Land Before Time book, "The Great, Beautiful Valley". When he and his friends are telling Littlefoot's Grandma what they would do if it weren't raining, Petrie says "Me fly to cliffs and watch Mountains That Burn."
Despite their somewhat similar appearence, the chances of the one shown in the picture here and the Fire Mountain are low due to the differing surroundings shown in that picture.