The latest release of Thunderbird (Version 13.0.1) has now been added to the two long-term support (LTS) versions of Ubuntu, namely: Ubuntu 10.04 (Lucid Lynx) and Ubuntu 12.04 (Precise Pangolin).
As previously advertised, the new version of the E-mail client features custom buttons to cycle through messages. In particular, there are both “Previous” and “Previous Unread” buttons, and the corresponding “Next” and “Next Unread” buttons, so that users can select graphical controls based on their specific preferences.
As noted earlier, this change has been a long time coming, but is nonetheless most welcome!
Thunderbird 13 Officially Lands in Ubuntu