The Smoking Mountains are the volcanoes in the background. From The Land Before Time IX: Journey to Big Water.
Smoking Mountain is the term used by dinosaurs in The Land Before Time for volcanoes. It can specifically refer to a certain volcano near the Great Wall and also to a group of volcanoes just outside the valley.
These volcanoes may be the same as the Mountains that Burn though this is unlikely as the Mountains that Burn are less steep and more violent, constantly dribbling lava while the Smoking Mountains release only streams of smoke.
The Smoking Mountain's first and largest appearance is in The Land Before Time VII: The Stone of Cold Fire. The Stone of Cold Fire crashed into one of the mountains, Threehorn Peak. When Pterano kidnaps Ducky and the others go to find her, they travel through the Smoking Mountains. When the gang found a large gorge, they used thick vines to cross. Unfortunately, halfway over, Spike munched on a flower sprouting from a vine, accidently biting a chunk off the vine so it broke. The group clung to the vine, and swung into a cave. In the cave, they found Ducky, who had escaped the Flyers. Unfortunately, upon exiting the cave Sierra swoops down and re-captures the Bigmouth. She bites his talon, causing him to drop her into the fast-flowing river at the bottom of the gorge. Using the vines (which had tangled around Spike) and a log, Littlefoot rescued Ducky, only for Cera and Spike to be dragged onto the river on pieces of wood with the vines in their mouths, similar to water-skiing.
The gang later get to the top of Threehorn Peak and the so-called Stone of Cold Fire at the same time as Pterano. The gang are rescued by Petrie's Mother and a Quetzalcoatlus and get away, along with Pterano, but Rinkus and Sierra are caught in the erupting volcano trying to get the Stone of Cold Fire's power. The Smoking Mountains' only other appearance is in The Land Before Time IX: Journey to the Big Water when it is stated that when the gang previously visited the Big Water they got back to the Smoking Mountains then turned right to get back home to the Great Valley and that they could find their way back from the Smoking Mountains. Mo's Water Kin called out and were told the direction by their Old One. They then pointed out the Smoking Mountains and the gang went to them and found the Great Valley as expected.