Applies to Thunderbird + Lightning Calendar Addon (v3.3.3)
(I was looking for that option somewhere in the calendar view, but couldn't find it anywhere)File > New > Calendar
For accessing a shared online calendar (e.g. iCal (.ics) File WebDAV link from OwnCloud), choose
"On the Network"
Then, select "CalDAV" and paste the URL to your .ics file CalDAV link.
The link might look something like this:
If everything went fine, that calendar should now show up in the left pane under "calendars".
(btw: I just found out, that right-clicking on one item in that calendar list also gives you the option to add/create a new calendar)
That should be it