Restrictions to all users into web login expect project owner

I am developing an admin website. To login into website I am only providing firebase login with email and password, no registration for new users. But the problem is every person in authentication are able to login into my website. I want to restrict all users except the firebase project owner. How can I handle that. For this I need to write code in Node.js(cloud functions) or I can write itself in web javascript and how?

Thanks in Advance.