If you are building a modern web application using Next.js 14 (App Router) and Firebase Authentication, you have likely encountered the dreaded "Text content does not match server-rendered HTML" warning. In severe cases, this escalates to a full crash: "Hydration failed because the initial UI does not match what was rendered on the server." This error usually appears the moment you try to conditionally render a UI element based on authentication state—like swapping a "Login" button for a User Avatar in your navigation bar. This guide provides a rigorous, production-grade solution to handle Firebase Authentication state in Next.js without breaking hydration, using TypeScript and the latest React patterns. The Root Cause: The Storage Gap To fix the problem, you must understand the mechanical conflict between Next.js Server Side Rendering (SSR) and the Firebase Client SDK. 1. The Server Snapshot When a user requests your page, Next.js renders the initial H...
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