DungeoneerBarry Thorne drank it to gain immunity to the fireballs in the Firebomb Room, making it easier for him to collect the witch amber that was there.
Barry drank two other potions during his quest, Speedwell and Fast Backwards, and also handled a Freezer potion (which quite literally got into enemy Hands).
A Fireproof potion is also mentioned in the Knightmare fanfic Everybody Has A Price as being part of Theodora Snitch's 'thieving kit'.