If you get a warning page on your Thunderbird home screen telling you that the version provided by Ubuntu will become obsolete on the 24th April 2012 then you should add the Mozilla team PPA repository and install the version from there:
sudo add-apt-repository ppa:mozillateam/thunderbird-stable
sudo apt-get update
sudo apt-get install thunderbird
sudo apt-get update
sudo apt-get install thunderbird
You will now have the latest version