There isn't really a concept of a "Folder" in either Google Cloud Storage or Firebase Storage.
Folders are simply common portions of a path that two objects happen to share.
For example:
gs://bucket/images/image1.png
gs://bucket/images/image2.png
There is no concrete folder object that represents "images". There are simply two objects whose paths have something in common (they are both under "/images/"). That said, the Firebase Storage SDK gives you some ways to organize your data into these "folders" via a StorageReference at that point.
StorageReference images = FirebaseStorage.getInstance().getReference().child("images"); <== represents the "images" part of the path for a future item.
images.child("image1.png").getBytes(...); <== downloads image1.png.
The SDK does not give any options to list these "folders" at this time. However, you can build your own directory structure using the realtime database. This has the benefit of realtime updates to clients when you update it. The only complexity here is that you'll need to keep the realtime database in sync as you add more items in storage.
-ben