A tea cup is a cup intended for drinking tea from.
A cup of tea is some tea placed in a cup for drinking.
Would you like another cup of tea?
usually means keeping the same cup but with more tea poured into it.
Would you like another tea cup?
means would you prefer a different piece of crockery, perhaps the first one is cracked.
If you go to a large department store and say you want twelve wine glasses you will be directed to the tableware department. If you say you want twelve glasses of wine you will be directed to the cafe area.
So "fire goblet" emphasizes the nature of the goblet itself, whereas "goblet of fire" emphasizes the contents of the goblet--the fire.