It has been a while since this question but I have found another solution that can help in this direction.
Contacting the support of Firebase, I was sent a URL that allows checking subdomains more assertively than the interface of Firebase.
The URL is https://www.google.com/webmasters/verification/verification-success?hl=pt-BR&domain=[seu_subdominio]
When you enter this URL, you will be driven to place a TXT code on your DNS provider, which is similar to what is done in Firebase. However, this check works and causes this subdomain to be recognized as yours across the Google platform (it seems).
After that, just go to Firebase, reload the page and re-request the domain inclusion. When entering the address, Firebase will already recognize that the domain is yours and provide the addresses for inclusion in DNS.
I have a .campinas.br domain that can be entered. However, I cannot confirm the address (after inserting in Registro BR). The confirmation screen understands that I am the owner of the campinas.br domain
– Victor Leal
With me happened the same thing, apparently they haven’t solved yet.. decides to host on github same
– Lennon S. Bueno
u put it on github and set the domain? are using firebase for other things yet? has you share the link here?
– Victor Leal
So I used gitihub only to host the site, and I continued using the Firebase database, and yes I got my domain .SJC.br for this required a configuration as I am on your mobile I have not easy but only search on Google that you find easy
– Lennon S. Bueno
cool!! but the link I referred to is the link of your app :)
– Victor Leal