There are two small icons beside the minimize, there is no tab unless you open on by clicking the icons.

Thanks Matt, but there is only one little icon next to the minimize and that one is for address book. Clicking on the gear icon in the add-ons manager/view recent updates shows that Lightning 4.0.1 is installed, there seems to be no way to make it visible.

Lightning just disappeared from Thunderbird after using it for several years.

It turns out that booting up in safe mode allowed Lightning Calendar to install.

Apparently the latest version of TB required an update to the newest version of Lightning.

Something in my computer was blocking that update, but re-booting in safe mode was the cure!!

Apparently the latest version of TB required an update to the newest version of Lightning.

This is always the case when Thunderbird does a major version change. Thunderbird and Lightning are matched versions. Maybe now that Lightning is included with Thunderbird it will become less of an issue.

Interesting that something on your computer was stopping the install.

Thanks for the update. It will help other with the same issue.