See the component page for more information.
Import files are a new invention, so libraries like firebase do not yet provide them.
firebase.html is an intermediary that provides an import file for the firebase component. firebase.html depends on firebase.
Components that want to use firebase should depend on firebase-element and import firebase.html 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.