The Firefly was similar to the Fairey Fulmar. The Fulmar had a second crew member because it was designed when carrier aircraft did not carry navigational aids to find the CV, so they put a navigator in it to help get home - whether that is true to the Firefly i do not know.
Debonair, I have read somewhere that it improved roll rate - unfortunately it did not specify by how much. If i remember rightly i think they clipped 8 inches off the wingspan