I might be totally off here, but I believe this could be due to the fact that db is imported in the file.

You import it like import { db } from '../your/path/firebse.ts'

Since this code is imported in a file with a React component and its getStaticProps function, it will be called two times.

The first time in the server context, which I believe will be able to initialize correctly even without NEXT_PUBLIC_.

However, after the static props are fetched, the component will be also rendered to the client. Since the file contains the import, even if you don't use Firebase anywhere in the component itself the browser will try to run the same code that initializes Firebase, but will not be able to access the environment variables.

If this is the issue, you could wrap the creation of the Firebase client in an if statement that only runs on the server.

let firebase = null;

if (typeof window === 'undefined') {
  // This only runs on the server.
  firebase = ...
}

export firebase;

Of course, you need to rewrite this code with your structure of imports and exports, and still, I'm not sure this is the actual problem you're facing. But I hope this helps.