while these are not specific to firefly,
bring a insulated cup and/or beer cozy if you drink.
Bring bug spray and anti itch cream
Bring a beach towel.
Bring small bills for tips for the bartenders and for house keeping. But you can also run a tab and tip at the end, which I just found out last trip.
For two bucks, you can buy postcards in the office and they will stamp and mail it for you.
Buy juice from both of the guys that walk by. Flex and I forget the other guys name but he has papaya-mango that is delish. Give them the bottles back when you are done.
try the jerk pork form the guy that walks by with the aluminum box.
get coco bread and a piece of cheese from the bread guy that walks by. A great snack for $2.
Be careful if you swim out past the markers, don't get hit by a jet ski.
Not really my thing but some people bring inflatable rafts.
Small gifts for the bartenders/maids etc. By the end of the trip, you'll be friends with them and something besides money is nice.