All email, calendar and contact clients that are to access Exchange Online need to have an approved enterprise app.
Admin Consent: If your organization restricts third-party apps, your administrator may need to grant permission via Microsoft Azure AD (Entra ID) for Thunderbird to access the account:
Together with the Postmaster group at my University we have approved some other applications than Outlook, Google Mail and Apple Mail, among them: Thunderbird and MS365 for Sailfish OS.