Canonical has added Thunderbird 31.7 in the default repositories of all the supported Ubuntu systems. Being a point release of Thunderbird 31, the latest version of the email client only adds security fixes and stability enhancements.
Before it gets pushed into the official repositories, the software receives fixes from Canonical and Thunderbird makes no exception. The Ubuntu developers had fixed 4 issues, before making it available for everybody. For information about this fixes, see this page.
Thunderbird 31.7 has been added to the default repositories of Ubuntu 15.04 Vivid Vervet, Ubuntu 14.10 Utopic Unicorn, Ubuntu 14.04 Trusty Tahr, Ubuntu 12.04 Precise Pangolin and is also available in the derivative systems sharing the same repositories: Linux Mint 17.1 Rebecca, Linux Mint 17 Qiana, Linux Mint 13 Maya, Pinguy OS 14.04, Elementary OS 0.3 Freya, Elementary OS 0.2 Luna, Deepin 2014, Peppermint Five, LXLE 14.04 and Linux Lite 2, so in order to install it, you only need to update the local repository index and install the thunderbird package:
$ sudo apt-get update
$ sudo apt-get install thunderbird