Mozilla Thunderbird 3.1 was just released featuring faster search, quick filter toolbar, better migration assistant and many other improvements. A complete list of what's new can be found @ Mozilla's website.
There is no PPA for Ubuntu providing the latest Mozilla Thunderbird 3.1 for now and there are no plans to push an update in the existing Ubuntu versions (that means Lucid too) like it happened with Firefox so the only way to install it is manually, or by using Ubuntuzilla (it works with any Ubuntu version).
So to install Thunderbird 3.1 in Ubuntu, firstly install Ubuntuzilla. You can download a .deb file from HERE.
Once Ubuntuzilla is installed successfully, to install Thunderbird 3.1, all you have to do is run the following command in a terminal:
ubuntuzilla.py -a install -p thunderbird
During the installation, Ubuntuzilla will ask you to choose the localization you want to install, so it will install Thunderbird 3.1 in your language!
If you used the manual method to install Thunderbird 3.1 we were telling you about not so long ago, then all you have to do is select "Check for updates" from the Thunderbird About menu.
Update: you can also download a Thunderbird 3.1 Ubuntu .DEB file!
Update 2: There is now a PPA repository for Thunderbird 3.1 available for Ubuntu (both 32bit and 64bit) users.
Thanks to Linux Freedom For Live