Mozilla released the next update for Thunderbird, version 31.1 on Tuesday, September 2nd. This release addressed security issues and included several bug fixes. See the release notes for more information.
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 in about six-weeks with Thunderbird 31.2.0 on October 14, 2014.