Domain Zoo is an aggressive deck with efficient small threats like Ragavan, Nimble Pilferer and Territorial Kavu backed by cheap removal and countermagic, so extra Unholy Heat gives us cheap answers to their early creatures, especially once delirium turns it into a 6-damage kill spell for bigger threats like Scion of Draco. Firespout clears their non-flying board (Ragavan, Territorial Kavu, Phelia) for 3 damage in one sweep, which is very efficient against their low-toughness creatures. We cut our narrow artifact/graveyard-hate slots since this matchup is about racing and removing their threats before they overwhelm us with Leyline Binding-backed pressure. Our own Eldrazi Spawn tokens and Basking Broodscale provide good blockers to buy time while we deploy removal.