The process to request to transfer an app ownership described at Transferring an app to another Firebase account is not working anymore, after send and email requesting the change I received the following message:
Hey there, In order to more efficiently process your issue, we now only accept new support requests through our support portal at https://firebase.google.com/support/. Please submit your inquiry there. Thanks in advance!
Which are the steps to doing it using Firebase Support portal?
Solution: I did a request on the new portal, under account services and a representative communicated with me through email to do the owner transfer. Thanks Frank van Puffelen!!
This process is actually a lot simpler nowadays. You can simply add a new collaborator to the project and make them an owner. See How to add collaborators to a Firebase app?