1. Head over to the Azure portal, log in with your Microsoft account, and pick the "Azure Active Directory" service.
2. On the left side menu, hit "App registrations" and then click the "New registration" button.
3. Fill out the necessary fields (Name, Supported account types, Redirect URI) and click "Register".

1. In the overview of your shiny new app, locate the "Application (client) ID" and the "Directory (tenant) ID" fields.
2. Copy these values because you'll need them later to set up Firebase.

1. On the left menu of your app, click "Certificates & secrets".
2. Click the "New client secret" button, fill in the required fields (Description, Expires), and click "Add".
3. Copy the Value of the new secret because you'll need it later for Firebase.

1. On the left menu of your app, click "Authentication".
2. Click the "Add a platform" button and choose "Web".
3. Set the "Redirect URIs" field to "https://project-id.firebaseapp.com/\_\_/auth/handler" (replace "project-id" with your Firebase project ID).
4. Set both "ID tokens" and "Access tokens" to "Yes" and click "Configure".

1. Go to the Firebase console, select your project, and click the "Authentication" menu.
2. Click the "Sign-in method" tab and enable the "Microsoft" provider.
3. In the form that pops up, fill in the fields with the data you got from the Azure portal: "Client ID" (Application ID), "Client secret", and "Tenant ID".
4. Click "Save" to lock in the changes.

Now, the Firebase Authentication service is hooked up with Microsoft Azure AD, and users can sign in to your app or service with their Microsoft credentials (whether personal or business).