Transferring an app to another Firebase account

@SergeyKopanev

Transfer the billing

After deleting the first user in the Firebase Console, login to your Google Cloud Platform console with your new user. Go to Billing > Account Management

Click on the three dots next to the project ID you just transferred, and select Change billing. Confirm by clicking on Set account and you are done !

Note that this is an old answer and there might be more up to date solutions below.

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This is a reply received after emailing to [email protected], which is the official firebase support email after gogle acquired firebase in late 2014.

From: [email protected]
In order to transfer ownership of a Firebase instance, we need you to do two things: add the new owner as a collaborator; then, fill in the form below and email it to us as written verification. This must be sent from the email address which owns the Firebase instance.

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REQUEST TO TRANSFER OWNERSHIP
I authorize the Firebase team to make **<new owner’s email>** the owner of https://**<firebase instance>**.firebaseio.com effective immediately. I have added **<new owner’s email>** as a collaborator to verify the email address is correct and to certify that I am the current owner. I understand that I am effectively granting **<new owner’s email>** full read and write access to all stored data for this instance.
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I recently shifted ownership of one of my projects to another account. All you have to do is:

1. Go to your Firebase console, and select the project you want to shift.
2. Select the cog icon besides the project name on top right.
3. Select Permissions from the flyout.
4. Select Advanced permission settings hyperlink.
5. You've reached the IAM & Admin page of Firebase.
6. Click on +Add button on top.
7. Enter the email ID of the account that you want to transfer the project to.
8. In the dropdown, Select a role > Project > Owner. Click add
9. You will receive a confirmation email. Click the link in the email to accept the invitation.
10. Accept the invitation, and go to IAM & Admin page of the transferred project.
11. Use remove button to delete the previous user

Hope this helps.