Elemental Plane of Fire
The Elemental Plane of Fire is the elemental fire plane of the Prime Elemental Planes. It is the ultimate source of beings such as fire elementals and wyrdlings. Very little seems to be established about it thus far. The Amber Flame society of Ta'Illistim walks the "planes" of fire, because there are no nearby volcanoes and what they can do in the city is limited, which may be equivalent or include the prime elemental plane of fire. For this to be literally possible it is presumably the case that "fire" somehow is constitutive of solid surfaces and forces such as gravity and that manifestations of flames are more symptomatic.
The Elemental Confluence
The Elemental Confluence is a composite realm of elemental planes merging, and so is not a pure representation of any given one of the Prime Elemental Planes. However, there are some regions of it that seem almost purely fire based, notwithstanding having ground to walk on and breathable air. The following are some examples of more or less pure elemental fire in the Confluence:
[Elemental Confluence - 23332] Enormous columns of fire billow up from the ground in huge pillars of churning flames. They bend and twist like fiery cyclones, their existence in a constant state of movement. Each burning pylon grasps up at the sky, resembling colossal red fingers trying to scorch the heavens. Obvious paths: north [Elemental Confluence - 23285] A fiery red-gold glaze is masterfully painted across the sky, the unnatural incarnadine clouds floating overhead looking like sinew and blood. Spindles of fire twist and churn, spiraling down to burn across the ground, leaving long and winding tracks of scorch marks. An amber haze drifts through the area, scattered to and fro by warm winds. You also see a point of elemental tranquility. Obvious paths: northeast, east