Fire-Resistant Clothes are first mentioned in Everblaze, when Sophie and Fitz went to heal Fintan in Oblivimyre.
They’re made of Flareadon and Bennu feathers: a fitted tunic and pants, knee-high boots, and a cape. Despite the clothes being fire-resistant, they do not protect the wearer against Everblaze. They also do not protect the person wearing them against extreme heat.
It can be noted that Sophie wished she had thought of using temperature control to cool herself down while she was wearing them.