Mozilla has pushed out an emergency update to Thunderbird 31.1.0 on Tuesday, September 9th. Thunderbird 31.1.1 addressed two critical bugs:
Depending on the users update settings, they will be prompted to install the Thunderbird Update or may do so via Help > About Thunderbird. Users can also download and manually install the latest version of Mozilla Thunderbird via getthunderbird.com site. The next scheduled update for Thunderbird will be Thunderbird 31.2.0 on October 14, 2014.