Magma heats up the creatures that live in it. I'd like to propose that the default body temperatures of any such creatures be lowered to the point they die if they ever reach it due to prolonged removal from their native element. Fire men burning out, magma men slowly solidifying and fire imps dying from the lack of heat would serve to explain why such creatures are never encountered outside their habitat and would add flavor to them. Fire men would last a relatively short time away from their source whereas magma men would take a long time to solidify, becoming increasingly sluggish as they do.
As for fire snakes, I have an idea. Who says liquid fire has to literally burn? It could alternatively be a substance with a shifting, fiery glow that absorbs heat and turns it into energy as nourishment for the critter. Treating protective clothing with it; using an alchemical process to fuse it's properties into silk, would provide protection against heat, something which would be invaluable if convection ever becomes a factor. Magma furnaces would be unbearable to work in without specially treated clothing which would help balance the lack of need for fuel magma access provides. It would also offer a counter to various heat-based enemies, some of which would realistically be unbeatable. The substance would likely lose it's radiance over time or with exposure to heat, requiring reapplying or replacement after a number of months or seasons have passed.
Additionally, I would hope that magma be made a little bit hotter than it realistically should be if we're going to maintain the ability to smelt any metal with it. As is, it just shouldn't be capable of doing so.
Thoughts?