The Brothers

Two mountain ranges run across the middle of the Firelands. These series of mountains are both extremely volcanic, with several peaks throughout them exhibiting smoldering, glowing calderas. The ash and heat from these series of volcanoes contributes heavily the nature of the Firelands. 

Twin Mountain Ranges

Named due to their similarities and close proximity - they are essentially the same mountain range with a large pass through the center - The Brothers are the result of an elemental hotspot that has remained under the Firelands since the fall of the Giants. After the elemental chaos unleashed by the sudden release of pent-up energies, several volcanic eruptions blew into the northeast corner of the continent. While these volcanoes have decreased in activity over the subsequent six thousand years, they are still regularly active, constantly spewing hot gas and ash into the atmosphere of the region. 

Big Brothers

The western range of the Brothers, "Big Brothers" are the most active and the most toxic. Producing heavy clouds of ash that subsequently rain upon the lowlands of the Firelands, the Big Brothers are nonetheless full of magical life. Phoenixes cavort in the skies, flitting around eruptive plumes; fiery imps and salamanders frolic in the calderas; earth elementals rumble down the slopes of the volcanos. The rare mortal visitor to the mountains would find themselves rapidly overcome with the aggressive reactions to outsiders that the natives to the region would have. 

When populating encounters in the Big Brothers, any creature with a Fire immunity would be at home here. Focus on those with a fiery appearance for maximum flavor, or feel free to reskin something to have an appropriate presence within the area.

Little Brothers

To the eastern edge of the Firelands, forming a border between the toxic ash-laden air of the lowlands and the humid, lush Nublar Jungle, the Little Brothers are somewhat smaller than the Big Brothers. In addition, they are much less toxic in their output. While still producing ash plumes that blow into the sky, the mountains themselves are full of steam vents that release heat that combines with the wet sea air blowing in from the eastern sea. The result is the moist heat that creates an environment perfect for enormous trees and rich life.
Life within the Little Brothers themselves depends on the slope. The western slope, facing the Firelands, has a similar wildlife population to that of the lowlands - many fire elementals, though generally of a less dangerous variety than those in the Big Brothers. The eastern slope has large trees growing up their slopes, from the jungle, and several species of Dinosaurs may be found there - especially flying varieties.