Mozilla released the final scheduled update for the Thunderbird 31 branch on Tuesday, March 31 with Thunderbird 31.6.0. Release notes.
Users may be prompted to update to the newest release of Thunderbird or can do so manually within Thunderbird by going to Help > About Thunderbird and following the update prompts. Users may also manually download and install the Thunderbird update via the getthunderbird.com site.
The next scheduled update for the Thunderbird will be May 12, 2015 with Thunderbird 38.0.