From the RuneScape Wiki, the wiki for all things RuneScape
A player wears the Fire tiara.
Fire Tiara is a type of Tiara combined with a Fire talisman, and is worn on the headwear slot. Fire Tiaras are used for crafting fire runes from rune essence. Wearing the fire tiara on the player's head allows the opening one inventory space. Thus, instead of being able to only take 27 rune essence to the Fire altar, the player may take 28 rune essence. It provides no combat bonuses.
A fire tiara can be worn in the head slot, allowing the player to left click on the altar to get inside, as well as saving an inventory space.