In DSA 3918 Thunderbird was upgraded to the latest ESR series. This update upgrades Enigmail, the OpenPGP extention for Thunderbird, to version 22.214.171.124 to restore full compatibility.
For the oldstable distribution (jessie), this problem has been fixed in version 2:126.96.36.199-1~deb8u1.
For the stable distribution (stretch), this problem has been fixed in version 2:188.8.131.52-1~deb9u1.
We recommend that you upgrade your enigmail packages.