You will have to ask Firebase support what they do in this case.
You will be able to issue a new certificate for the domain using Certbot once you have adjusted the DNS records away from Firebase.
RE: Revocation, my advice is to not bother in this case unless you believe the private key were compromised somehow (e.g. lost by Firebase or something). The certificate will expire in a short time anyway and revocation is largely a flawed process. ACME allows you to revoke the previous cert by proving ownership of all of the names that were on the previous certificate with a new account and then requesting the previous cert be revoked using the new account. I’m not sure if Certbot implements this or how to achieve it, again I recommend not to bother.