Log into your console and in the project you have created there is an option to add a custom domain. In the Firebase console choose option to create custom domain and enter your website details. Do not include https etc.
Verify ownership of the domain name via a TXT DNS record choose the advanced option. Copy the text value and create a TXT record with your domain provider. You can leave this screen open while you do it. After you have created a text record choose the option to verify ownership. You cannot proceed until verification is complete.
After you have completed this progress Google will issue your SSL certificate attached to the custom domain. However this process is not instant and custom domain will have a status Pending. Until the certificate is issued you will not be able to browse the site. If you get the view hosting documents page when you try to view your site. This is the default index.html that Google has created. You need to redeploy the site to replace this page once the custom domain is showing as connected in the Firebase console.