When this face-up card on the field attacks or is attacked and it is destroyed as a result of battle, the player who destroyed it takes 1000 points of damage.
Rulings
Individual Card FAQs
The card text is very correct on the card mechanics; "Atomic Firefly" has to be face-up BEFORE the attack is declared.
"Atomic Firefly"'s effect activates in the Graveyard when "Atomic Firefly" is sent to the Graveyard at the end of the attack, so "Skill Drain" will not negate its effect.
You can use "Barrel Behind the Door" against "Atomic Firefly"'s effect.
LIGHT|
Level 1|[ Insect / Effect ]
ATK 100|
DEF 200