You could easily install Thunderbird 3.1 in Ubuntu using Ubuntuzilla, but that was only available for 32bit users.
A PPA repository is now available which provides the latest Thunderbird 3.1.1 (final, not daily builds) for Ubuntu Lucid - both 32bit and 64bit.
To add the PPA and install Thunderbird 3.1.1 in Ubuntu Lucid, simply paste the following commands in a terminal:
Reference:sudo add-apt-repository ppa:ricotz/ppa && sudo apt-get update sudo apt-get install thunderbird
http://www.webupd8.org/2010/07/thunderbird-31-ubuntu-ppa-repository.html
