Thunderbird uses the IMAP protocol to work with mail on Outlook''.''com. In order to sync with an Outlook''.''com ''calendar'' Microsoft requires using Exchange ActiveSync, which is not built into Thunderbird. There is a non-free add-on which adds Exchange ActiveSync capability to Thunderbird that you can check out if no one else has an easier/cheaper suggestion: https://addons.mozilla.org/thunderbird/addon/exquilla-exchange-web-services/