Mozilla has skipped version 4 and introduced Thunderbird 5.0 Final at last. You can find more information and download from its homepage http://www.mozilla.org/en-US/thunderbird/
Thunderbird is a free email application that’s easy to set up and customize – and it’s loaded with great features!
To install Thunderbird 5.0 Final in Ubuntu, there’s a simple way to do through PPA
sudo add-apt-repository ppa:mozillateam/thunderbird-stable
sudo apt-get update
sudo apt-get install thunderbird
Before installing, be sure you have uninstalled previous version to avoid conflicts & errors


