If a monster is FIRE because it is equipped with “Scroll of Bewitchment”, you cannot activate “Backfire”’s effect when it is sent to the Graveyard because it is no longer equipped when it arrives in the Graveyard.
When multiple FIRE monsters are sent to the Graveyard simultaneously, the damage is only 500 points.
If you control “Backfire” and your opponent’s FIRE monster, you can activate “Backfire”’s effect when your opponent’s monster that you control is destroyed.