Some creatures live and swim in magma. Is it possible to convince fire immune citizen that they can path through shallow magma like they do with water?
I doub't it's possible; Even then it probably wouldn't be a good idea since the stuff your fireimmune creatures are going to be carrying/wearing is most likely not magmaproof.
Hauled stuff burn, but worn or held does not. I supposed that they take on the temperature of their wearer.
I added fireimmune_super and dragonfire to my little steel angel chicks, had a companion with me and was running around laughing as I barfed fiery death everywhere (no wonder dragons do this, it's fun as hell!) the only thing she got stuck by was the whole flying creatures stuck in trees bug. She'd happily play with me in the middle of dragonfire without worries.
I did notice that at one point she had socks on along with her armor (no other clothes, they're pretty random about when they wear clothes properly) and a pouch with stuff in it, said stuff lasted the first several runs through fire, but later we took on a hydra and as I was a wrestler it took a while so I just said screw it and melted it with firespam. Later I noticed that she no longer had her cloth items with her, so I assume they burned.
I've had cloaks catch on fire and tossed them away from me, and I've had an adventurer with a masterwork copper spear--which he grabbed from a goblin and kept with him after getting some scars--strapped to his upper body that turned a corner into a cave to see a dragon yawning at him. I jumped out and tried to jump away to maybe find water and see if that would help before I ended up dying. I checked legends to confirm where the lair was and saw that he was listed as being killed by that copper spear, not the dragon! It melted and he bled out.
Oh, and I did not know there were fitting tolerances beyond very small/small/large/huge gear and the respective races.