It should be listed in the add-ons. It is here. I am pretty sure it is
because the "xul-ext-lightning" package is installed. That said I guess you
could as well install the Lightning extension from this file (as you did for
CardBook):
I actually remember that, right after installing Trisquel 8, I add the
calendar docked in the right-hand side of Icedove's main window. But since I
do not use that feature, I disabled it. Re-enabling it, I have two buttons
at the right of the tab bar to "Switch to the calendar tab" and
Lighting is in the default install (the related package is
"xul-ext-lightning"). You can download
https://addons.thunderbird.net/thunderbird/downloads/latest/cardbook/addon-634298-latest.xpi
and install this file from the "gear" menu in the window listing the
installed extension
