Even though the second step's (Review APIs enabled and resources created) Next button is disabled, I can click on the third step, and its Next button is enabled. But when I click it or the fourth step (Configure Extension), nothing happens. No configuration options are displayed.
This seems to happen when Storage is not enabled in your Firebase project. So go to the Storage tab in the Firebase console, and complete the initialization there - even if you don't (want to) use Cloud Storage in your Firebase project.
This provisioning of Storage used to happen automatically when you'd create a new project, but is now only done on demand.