The problem is worse than what I described above: the thunderbird-locale-fr is correctly installed, but Thunderbird does not seem to see it.
The real workaround: cd /usr/lib/thunderbird sudo rmdir extensions sudo ln -s ../thunderbird-addons/extensions .
So the real problem is that the thunderbird installation did not replace the directory /usr/bin/thunderbird/extensions by the symlink.
The problem is worse than what I described above: the thunderbird- locale- fr is correctly installed, but Thunderbird does not seem to see it.
The real workaround: thunderbird addons/ extensions .
cd /usr/lib/
sudo rmdir extensions
sudo ln -s ../thunderbird-
So the real problem is that the thunderbird installation did not replace the directory /usr/bin/ thunderbird/ extensions by the symlink.