Thunderbird is no longer developed by Mozilla.
https://blog.mozilla.org/thunderbird/2017/05/thunderbirds-future-home/
github.com/thundernest does not contain source code of Thunderbird client
> comm-central is the main Thunderbird repository.
> Thundernest contains the code for Thunderbird.net (this site),
as well as the Thunderbird web server setup scripts.
from https://www.thunderbird.net/en-US/get-involved/
In _includes/cardv2.html add `source`, because git is not the only
version control available (there is Mercurial, Subversion, CVS)
