To add your ownCloud calendar to Thunderbird you need the “Lightning” add-on for Thunderbird. The add-on is already installed by default, but can be added manually under “Add.Ons” if necessary.
Go to the calendar tab by clicking on the tab that is already open or by clicking on “Switch to calendar tab” in the top right corner.
Add a new calendar by clicking on the left side below the existing calendar on the right and clicking on “New Calendar …” in the context menu. In the following window select “In the network” and click on “Next“. Now select “CalDAV” and under “Address” enter the link to your calendar.
Confirm the settings and click “Next“. In the next window you can give your calendar a name and with a click on “OK” the calendar will be added.