See the component page for more information.
Import files are a new invention, so libraries like firebase do not yet provide them.
firebase-import is an intermediary that provides an import file for the firebase component. firebase-import depends on firebase.
Components that want to use firebase should depend on firebase-element and import firebase-import to be safe from library duplication.
Such components need not use Polymer or firebase-element, but we put the import and the element in one package for convenience.