If you are referring to verifying the email address, you can check this documentation on how to do so [1].
If what you need to verify is the bank account associated with your billing account, then you will find the information in this other link useful [2].
You can also use this Firebase billing troubleshooting tool [3] to contact the billing team.
[1]: https://cloud.google.com/billing/docs/how-to/manage-billing-account#verify_your_email_address
[2]: https://cloud.google.com/billing/docs/how-to/verify-bank
[3]: https://firebase.google.com/support/troubleshooter/billing