Activate Fanatical Firebrand by tapping and sacrificing it.
When Fanatical Firebrand dies, Luminous Broodmoth triggers, returning Fanatical Firebrand from your graveyard to the battlefield without a flying counter on it due to Solemnity.
Resolve the Fanatical Firebrand ability.
Repeat.

Infinite death triggers

Infinite creature ETB

Infinite creature LTB

Infinite creature sacrifice triggers

Infinite damage

Solemnity

Luminous Broodmoth

Fanatical Firebrand

Cast Fanatical Firebrand by paying its mana cost.
Activate Fanatical Firebrand by sacrificing it.
When Fanatical Firebrand dies, Mortuary and Sifa Grent trigger.
Resolve the Mortuary trigger, putting Fanatical Firebrand from your graveyard onto the top of your library.
Resolve the Sifa Grent trigger, causing target opponent to lose 1 life and you to gain 1 life.
Cast Fanatical Firebrand from the top of your library by paying 1 life.
Repeat from step 2.

Infinite lifegain triggers

Infinite lifeloss

Infinite storm count

Mortuary

Bolas's Citadel

Sifa Grent

Activate Fanatical Firebrand by tapping it and sacrificing it.
When Fanatical Firebrand dies, its persist ability triggers, returning it from your graveyard to the battlefield without a -1/-1 counter on it due to Vizier of Remedies.
Resolve the Fanatical Firebrand ability.
Repeat.

Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight

Vizier of Remedies

Activate Fanatical Firebrand by tapping and sacrificing it.
When Fanatical Firebrand dies, Yedora triggers, returning Fanatical Firebrand from your graveyard to the battlefield face-down as a Forest land.
When Fanatical Firebrand enters the battlefield, Zimone triggers.
Holding priority, activate Fanatical Firebrand by tapping it, adding {G}.
Resolve the Zimone trigger, turning Fanatical Firebrand face-up.
Resolve the Fanatical Firebrand ability from step 1.
Repeat.

Infinite green mana

Infinite landfall triggers

Zimone, Mystery Unraveler

Yedora, Grave Gardener

Activate Fanatical Firebrand by tapping and sacrificing it.
When Fanatical Firebrand dies, Ratadrabik triggers, creating a token copy of Fanatical Firebrand that is a 2/2 legendary Zombie in addition to its other types.
Resolve the Fanatical Firebrand ability.
Repeat using the Fanatical Firebrand token

Ratadrabik of Urborg

Leyline of Singularity

Combos with "Fanatical Firebrand"