Comment 5 for bug 995054

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.