Comment 12 for bug 1797945

Recent versions of Thunderbird (at least the version packaged / distributed by Mozilla) has come pre-installed with Lightning; this is why the extension hasn't been updated on addons.mozilla.org.

The version of Thunderbird installable from the standard Ubuntu repositories is packaged by Ubuntu. They have chosen to package the extension as a separate package (xul-ext-lightning), so the most "correct" solution to this issue is to install that package.

(xul-ext-lightning being English-only appears to be long-standing issue (#545778); if this affects you I suggest contributing to that bug report.)

If you would still like to install the .xpi file (perhaps because of the locale issue):

1. Visit the Thunderbird site (https://www.thunderbird.net/) and download the Linux / Linux 64-bit version of Thunderbird, matching your installed version (60.2.1 in this case) and locale. It should be a .tar.bz2 archive.
2. Extract the archive, then inside thunderbird/distribution/extensions there should be one .xpi file. That should be the corresponding version of Lightning (6.2.2.1).