Lightning is a free calendar plugin for the free Thunderbird email client.

To add a VPOP3 CalDAV calendar to Mozilla Lightning is straightforward

1. Install the Lightning plugin for Thunderbird if you have not already done so
   1. In Thunderbird, go to Tools → Add-ons
   2. Click Get Add-ons
   3. In the search box at the top-right, enter Lightning and search.
   4. Select the Lightning plugin and press the Install button
   5. Restart Thunderbird when prompted
2. Add the calendar to Thunderbird
   1. In Thunderbird, go to File → New → Calendar
   2. Select On the Network, press Next
   3. Select CalDAV, in the Location box enter the URL for the calendar. For the user's default calendar, this will be http://<server address>:5108/calendar/<username>/default, eg http://192.168.1.100:5108/calendar/postmaster/default/. Press Next
   4. Enter a name (for your reference), colours, etc for the new calendar, and press Next
   5. Press Finish
   6. Now the calendar should appear if you go to Events and Tasks → Calendar. If you have multiple calendars defined in Lightning, then you can select which ones are displayed using the list on the left of the page