I'm trying to add a custom domain to an existing project but keep getting the error that "this domain is already in use." I'm not using this domain anywhere else in any other project.
There is only 1 other forum post (http://grokbase.com/t/gg/firebase-talk/165r5n78nq/firebase-domain-is-already-in-use) about this error but following the step provided leads to zero results.
If you are getting that error it means that you're already using the domain in a project. You probably deleted the project but did not disconnect the domain before it was shut down. In this case, you can either wait for 10 days from when you deleted the project for it to be removed or, you can restore the project by going to the Firebase IAM & Admin console click on projects pending shutdown.
Then proceed to disconnect the domain from the project like you normally would. Now you can use that domain again with no problem.
Go to https://console.firebase.google.com/iam-admin/iam/project and click on 'Projects Pending Deletion" click Restore. Then go to https://console.firebase.google.com/project/YOUR-PROJ-ID/hosting/main and delete the domain connection(s).
That should free the domain up for a new SSL cert.
来源：https://stackoverflow.com/questions/37951909/how-do-i-remove-this-error-in-firebase-this-domain-is-already-in-use