Fire Giants drop three kinds of bones. No matter where you kill them, you can get these as a drop. Big Bones - This is a 100% drop of the fire giants. It is not worth very much but it could be useful if you pick them up and bury them in the catacombs of Kourend. If you do this you will automatically regain 2 prayer points. Depending on your prayer bonus and combat level these 2 points could sustain you indefinitely. All you need to do is kill them faster than it takes you to lose two prayer points.  Long bones - This is a pretty rare drop. Only 1 out of 400 will drop these. You can sell them for a 1000 coins, bury them or you can give them to Barlak and he will give you a lecture about construction, which will grant you 4 500 construction experience.   Curved bones - these bones are an even rarer drop in Old School RuneScape. You have a less than one in 5000 chance to get these. You can trade these in for 6 750 construction experience.  These are not unique drops to the fire giant boss, there are a couple of monsters that drop them.