Since December 2004, the calendar trunk (not 0.2 and its branch, specifically) has a more complete separation of calendar operation logic and storage/protocol operations, permitting a wider variety of calendar types to be supported.
Providers in the Mozilla Calendar tree currently provide support for [CalDAV], a local sqlite-based store (Calendar:Sql Calendar Schema), ICS files (local and "webcal"), and a simple in-memory store for testing and utility use. The source for these providers can be found at http://lxr.mozilla.org/mozilla/source/calendar/providers/.
There is a specific build that enables the provider for Sun Java System Calendar Server (WCAP).
Other providers have been developed as extensions, specifically the Provider for Google Calendar.
More providers could appear over time, although it will probably be up to whoever has the motivation and resources to develop them, which is not to say that such providers would be rejected. There is certainly user demand for Exchange and other calendar protocol implementations, and high-quality work in those areas would be welcomed very warmly by the calendar team and user community.
It is very much the intent of the current (underdocumented) provider design that additional provider types can be well-integrated via extensions to Calendar applications (including a Lightning-enabled Thunderbird in that set).