After recently upgrading from Dapper Drake to Hardy Heron I noticed that the Thunderbird Lightning calendar add-on wasn't working. I could see that it was installed but the calendar toolbar was not available.
The upgrade had installed Thunderbird 184.108.40.206 and I had installed the latest available version of the extension (0.8) directly from Mozilla.
Ultimately, I discovered that several files (in $HOME/.mozilla-thunderbird) related to the Lightning extension were owned by root. The fix was simply change ownership to me.
sudo chown -R my_user_id:my_group_id *