What do I do in the fire realm?

First, take a look at what the tree is doing.

It's touching the crystals.

But it's not the only thing on the screen that can touch the crystals.

Have you tried clicking on the sun?

When you do, it will shoot bolts of lightning at the crystals as well.

Click on the sun a few times, and you should notice that the sun will do
something else as well.

As long as your timing is right.

Whenever the tree and the lightning hit the crystals at the same time, a note
will play.

The lightning shoots out at all the crystals in a particular order.