Canonical has added Thunderbird 31.8.0 to the default repositories, despite the fact that the latest version available of the email client is Thunderbird 38.1.0.
It has been a while since Canonical updated the Thunderbird version from the default repositories of all the supported Ubuntu systems, but today, this has changed.
Despite this update, Ubuntu is the only Linux system that ships Thunderbird 31.x in the default repositories.
This being said, if you still to get the thunderbird version from the repositories, do:
$ sudo apt-get update
$ sudo apt-get install thunderbird