The effect of “Firebird” is a Trigger Effect that activates when 1 or more Winged Beast-Type monster(s) on the controller’s side of the field are destroyed. If the monsters were destroyed as a result of battle, the timing of this effect is at the end of the Damage Step, when the monsters are sent to the Graveyard.
You can chain to the activation of “Firebird”’s effect, including with “Divine Wrath”.
If 2 or more Winged Beast-Type monsters on the controller’s side of the field are destroyed at the same time, the ATK of “Firebird” only increases by 500 points.
Winged Beast-Type tokens on the controller’s side of the field are included and will increase the ATK of “Firebird” when destroyed.
Monsters on the controller’s side of the field that are changed to Winged Beast-Type by “DNA Surgery” are included and will increase the ATK of “Firebird” when destroyed.
When face-down Winged Beast-Type monsters are destroyed (by a card effect), the ATK of “Firebird” does not increase.
The ATK increase from the effect of “Firebird” remains as long as “Firebird” remains face-up on the field.