I’ve been an advocate of making Calendar an Extension for Thunderbird. I think Calendar extension is important in a Mail client, as most business users are accustomed to this (Microsoft Outlook). As a result, it makes sense for Mozilla to offer such functionality.
Besides matching, it’s also quite convenient. What 2 Apps do most people keep open? Email, and a Calendar. Secondly, Calendar can integrate quite well with email (Appointment notification, etc.)
This really is a giant step in bringing Mozilla closer to the workplace.