Comment 6 for bug 1797945

Same Problem today after "sudo apt upgrade" => from Thunderbird 52 to 60.2.1 (64-bit)
In all my 3 Profiles Lightning had stopped working / disappeared.
The First Profile had Lightning 5.4 installed as Add-On and now showed: incompatible .. deaktivated
Second and third Profile: In the Past, Lightning had been automatically integrated by clicking "keep it" and now it had disappeared.
Trying to install via search Add-Ons in Thunderbird only led to the incompatible 5.4.

Also NOT helpful was:
(Entry in Config did not exist at all)

There was also no change, when i started Ubuntu-Languages or so (DE: Sprachen) and let install additional Language-Packages including one for Thunderbird-EN automatically. (DE, which i use, did not show up as missing).

What then helped in all 3 Profiles, was the manual Download and Install of a Lightning-Beta-XPI into Thunderbird: