Fur editing does not work in prefab mode.

When using the prefab editor, the FurContainer needs to be exported to a file before you can use the prefab editor. So add fur in the scene view first: https://app.gitbook.com/s/-MfXVuHpudI1ZxHdYnQQ/~/changes/0RvrFH0U1iOaREMYmP2M/export/export-groom-to-file then create a prefab out of it. After that you should be able to edit the fur in prefab mode as well.