Senior Firebase Auth Developer — Fix Login Redirect Loop (Next.js)

Hi — I'm building a SaaS web app (Next.js + Firebase Auth) and I'm stuck on one bug I can't crack. When a user logs in, it just loops right back to the login page — the session never sticks. I've tried everything I know and need a senior developer to get me past it so I can move the project forward. It's well-isolated to the authentication flow, so this should be a contained job for someone experienced. I can share the repo and Firebase access right away, plus the exact error I'm seeing. I'm sure this a light change that is beyond vibe coder skills