In our Firebase project, we've enabled Microsoft as an IdP. After configuring the app in Azure and creating a Microsoft Business account, we can successfully login (using the popup method):
const provider = OAuthProvider('microsoft.com');
provider.setCustomParameters({ prompt: 'select_account' });
await signInWithPopup(auth, provider);
However, Firebase then returns the following error: auth/account-exists-with-different-credential, although there is no existing account with that email address.
Interestingly, this seems to happen only in projects for which we have upgraded to use Google Identity Platform.