Open Terminal and copy folder ~/.mozilla-thunderbird to the new location expected by Thunderbird 3:

copy ~/.mozilla-thunderbird ~/.thunderbird

All emails, settings and extensions in ~/.mozilla-thunderbird will be your backup.

Get Ubuntuzilla package signing key to your keyring:

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com C1289A29

Add Ubuntuzilla repositories to the list of your sources either by using main menu System > Administration > Software Sources on the Other Software tab:

deb http://switch.dl.sourceforge.net/project/ubuntuzilla/mozilla/apt all main

Hit Reload button when asked to update software list.

Alternativelly the above step can be done with the following two commands in your terminal:

echo -e "\ndeb http://switch.dl.sourceforge.net/project/ubuntuzilla/mozilla/apt all main" | sudo tee -a /etc/apt/sources.list > /dev/null
sudo apt-get update

And finally you can install the latest version of Thunderbird by running the following command:

sudo apt-get install thunderbird-mozilla-build

Don’t forget that you can erase the folder ~/.mozilla-thunderbird if the upgrade was successful. Have fun using the best email client in the world – Thunderbird 3! You can find it in the main menu Applications > Internet > Mozilla Build of Thunderbird.