Downgrade Linux Thunderbird
For those who have upgraded to latest Thunderbird to find Calendar add-on is suddenly not compatible. The quick fix is to roll-back to last version as follows:
Display Thunderbird available versions
apt-cache show thunderbird | grep Version
Select last version to rollback to, e.g.
sudo apt-get install thunderbird=1:52.9.1+build3-0ubuntu0.14.04.1