The Firium Z (Japanese: ホノオＺFlame Z) is a type of Z-Crystal introduced in Pokémon Sun and Moon. It is the type-specific Z-Crystal associated with the Fire type and allows Pokémon to upgrade their Fire-type moves to Z-Moves.
In A Timeless Encounter!, a young Professor Kukui was revealed to own a Firium Z, which he briefly lent to Ash for him and his Torracat to show him Inferno Overdrive. Later in the episode, Ash received his own Firium Z from a TotemTrevenant after helping to remove the Cutiefly that had gotten stuck in its branches, with Kukui revealing that he had obtained his Firium Z from it as well.
A full set of replicas of the type-specific Z-Crystals, including the Firium Z, appeared in A Team-on-Team Tussle!, where they were shown to Ash and his classmates by Professor Kukui.