Well, at least Google Cloud and Firebase in this case have the option to add additional users under a given role. In this case you would likely want the client to be Owner of the project, and ask them to invite you as a developer, this way you can sign in with your own Google account (either gmail or your personal if you use google workspace), hence bypassing the need for 2FA.
In general, there are things that you don't really want access to as developer, such as billing information, so it makes sense to ask for limited access. Both Firebase and GCP have predefined roles for this, you can do some research on them to find out which makes the most sense for you and advice your client.
Read more about the roles here:
GCP: https://cloud.google.com/iam/docs/understanding-roles
Firebase: https://firebase.google.com/docs/projects/iam/roles
Side note:
If you are working as a freelancer, I would recommend to never create accounts for your clients, if you are working with specific service, ask your client to setup the account (this will primarily protect you, if the client backs out or the server costs for development build up if the project is taking long time to finish)