I just completed the Fire Temple.
I headed over to Zora's domain and see that everything is frozen over. At this point, I am not sure what to do.
You need the blue fire, which you can use to unfreeze king Zora. Go past the frozen King to Jabu Jabu's area - there will be ice platforms you can jump from to make it to the island you saw in the area when you were a kid.
Good luck in the Ice Cavern!