There is a calendar extension available but it is not a production release.  Feel daring?

From http://wiki.mozilla.org/Calendar:Lightning:0.1:Release_Notes

What is Lightning 0.1?

Lightning 0.1 is an embedding of the Mozilla calendar code specifically designed to run inside Mozilla Thunderbird 1.5 and 1.5.0.*. This release is a preliminary, non-production release designed to showcase the work that's been done and allow contributors to help us test it out. True email / calendar integration is planned, but that is not part of this release.

What can Lightning 0.1 do?

• Help you keep track of your personal schedule
• Subscribe to calendars on the Internet (via HTTP/WebDAV)