After a standard system upgrade you need to restart Thunderbird to effect the necessary changes.
USN-927-1 fixed vulnerabilities in NSS. Due to upstream changes in NSS 3.12.6, Thunderbird would be unable to initialize the security component and connect with SSL/TLS if the old libnss3-0d transition package was installed. This update fixes the problem.
We apologize for the inconvenience.
Original advisory details:
Marsh Ray and Steve Dispensa discovered a flaw in the TLS and SSLv3 protocols. If an attacker could perform a man in the middle attack at the start of a TLS connection, the attacker could inject arbitrary content at the beginning of the user's session. This update adds support for the new new renegotiation extension and will use it when the server supports it.