After a standard system upgrade you need to restart Thunderbird to effect
the necessary changes.
USN-927-1 fixed vulnerabilities in NSS. This update provides the
Thunderbird update to use the new NSS.
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.