Mozilla Thunderbird is a total redesign of the Mozilla mail component to produce a cross-platform, stand-alone mail application using the XUL user interface language. It has many new features, among them the ability to customize your toolbars the way you want them. a new look and feel with a large number of downloadable themes which alter the appearance of the client, and the ability to add UI extensions.
What is new in version 3.1.3
You can check here for complete release notes
Install thunderbird 3.1.3 in Ubuntu 10.04
Open the terminal and run the following commands
sudo add-apt-repository ppa:lucid-bleed/ppa
sudo apt-get update
sudo apt-get install thunderbird thunderbird-locales