D&D is not a physics engine
Some rules, because they seem to simulate little bits of reality, might give you the impression that this is major point of the game. However, D&D reality is very skewed in favour of making games and fantastical stories. Neither games, nor fantasy novels nor cinema pay much more than lip service to science-based reality. This extends to pretty much all role-playing games - some try harder than others, but all ultimately fail, and require either DM rulings (of course subject to DM's understanding of reality) or accepting "it's just a game".
The D&D world breaks many laws of physics
Many game effects appear to break core conservation laws, such as conservation of energy - although you can posit magical energy of various kinds, I doubt you could really fix that. A lot of the science-based effects (e.g. Time Stop) are based on fantasy science that really doesn't work if you think too hard about it (and I'm not talking about impossibility of the spell - think for a moment what is happening to the atoms and photons in a Time Stop zone - how could you see anything in it?)
Fire immunity is an example of introducing an "immovable object", and your plasma idea is the "unstoppable force". This was an old philosophy conundrum when the two met, but modern physics would consider either concept as a nonsense.
The situation is unlikely to come up unless you construct it
Perhaps there is a magical trap that the players could find out about that generates a metal plasma, and it can be used to effectively petrify Fire Immune creatures caught in it. It might be a fun scenario to play. The chances of it happening without the players and the DM actively making it happen are unlikely though.
More mundane "reality" questions pop up now and then, when rules either don't exist, or seem to clash with real-world sense. You can rule these how you like, but I would advise to err on keeping the game and story alive as a priority over your sense of "reality".
Rule weird or unusual situations how you think they should work
A plasma of super-heated metal is a little out-of-canon for D&D, although if you find it fun, why not go for it. There is no RAW
On a simpler variant, I would rule a Fire Immune creature could drown in lava, much the same as a more normal creature would drown in water. There may even be RAW for this, although I haven't checked.
Double-check a few things afterward
If you enjoy exploring unusual situations in the game, you may want to check:
Other people in your group also find them fun
That you have not created an easy win option that would change the game and make it boring (all Fire Immune monsters can be defeated with the new tungsten plasma wand)