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.