Building secure applications involves robust authentication and authorization mechanisms, and this guide explores integrating Firebase and Permit.io to achieve this. Firebase offers a straightforward approach to authentication and data management but struggles with complex permission systems like role-based access control (RBAC) and relationship-based access control (ReBAC). To address these limitations, the guide demonstrates how to combine Firebase Authentication and Firestore for managing user identity and data, while using Permit.io for fine-grained authorization to enforce rules based on roles and relationships. The technologies are integrated into a Next.js task management app that supports multi-tenancy, allowing users to join multiple organizations and manage tasks with defined access controls. The guide details setting up Firebase for authentication and storage, creating structured Firestore collections, and integrating Permit.io for advanced access control, including defining resources, roles, and relationships. It also covers implementing API routes for role assignment and permission checks, ensuring that only authorized users can perform specific actions within the app. The solution enhances security and maintainability by separating authorization logic from database rules, offering a scalable and flexible access control model beyond the capabilities of Firebase's built-in security rules.