I've realized that, as Firefox, Thunderbird points the users to
addons.mozilla.org for extensions, a site listing non-free software.
This must be avoided redirecting the users to a list of free add-ons.
This can be easily fixed by changing some default config chains:
$ cat /etc/thunderbird/pref/freedom-settings.js
// Preferences for the Get Add-ons panel
pref ("extensions.getAddons.showPane", false);
pref ("extensions.getAddons.browseAddons",
"http://www.gnu.org/software/gnuzilla/addons.html");
pref("extensions.getAddons.maxResults", 5);
pref ("extensions.getAddons.recommended.browseURL",
"http://www.gnu.org/software/gnuzilla/addons.html");
pref ("extensions.getAddons.recommended.url",
"http://www.gnu.org/software/gnuzilla/addons.html");
pref ("extensions.getAddons.search.browseURL",
"http://www.gnu.org/software/gnuzilla/addons.html");
pref ("extensions.getAddons.search.url",
"http://www.gnu.org/software/gnuzilla/addons.html");
pref ("extensions.getMoreExtensionsURL" ,
"http://www.gnu.org/software/gnuzilla/addons.html");
pref ("extensions.getMorePluginsURL" ,
"http://www.gnu.org/software/gnuzilla/addons.html");
pref ("extensions.getMoreThemesURL" ,
"http://www.gnu.org/software/gnuzilla/addons.html");
// Dictionary download preference
pref("browser.dictionaries.download.url", "http://gnuzilla.gnu.org");