I found the solution yesterday after trying the methods mentioned here and in a couple of other places.
1. In your browser, login to iCloud and open Calendar.
2. Click on the little greyed-out "Share Calendar" icon beside the name of any calendar you want to view in Thunderbird/Lightning.
3. There are 2 options - Private and Public. Select Public.
4. A URL will be provided for anyone you want to share your calendar with. It starts with "webcal" instead of "https". Select and copy this URL.
5. Go to Thunderbird/Lightning and create a new calendar. (File->New->Calendar or just right-click an existing calendar and select "New Calendar" from the pop-up menu.)
6. Create the calendar "On the Network" and click "Next".
7. For "Format" select "iCalendar (ICS) and paste the sharing URL from #4 into the "Location" box. Click "Next"
8. Give your calendar a name. Select the colour and update frequency that you want. Click "Next".
9. Click "Finish".
10. Repeat for each calendar you want to view in Thunderbird/Lightning.
Publicly subscribed calendars are supposed to be read-only. I haven't tried to add any events on T/L yet (am having issues with clearing reminders that keep popping up and slowing things down, which I'm going to investigate next) but even though it's possible in T/L to unclick "read-only" in the calendar's properties (right-click) , a little lock keeps reappearing beside some of the calendars on my list so I'm not really expecting to be able to do so.
I hope this works for all of you too!
Erusa