According to a recent message posted on the Ubuntu Devel mailing list by Colin Watson, Installer Team leader, Ubuntu might get a new, simplified packaging format and app installer which should make it easier for developers to get their software into Ubuntu. This will target, at least initially, the Ubuntu phone/tablet but it should be usable elsewhere too, even on non-Ubuntu or non-Linux systems.

If you found the Ubuntuzilla way of installing Thunderbird 3.1 in Ubuntu a bit too hard, here is a .deb file you can download and install Thunderbird 3.1 in Ubuntu.

Note: this is the unmodified Mozilla release binary of Thunderbird, packaged into a .deb by the Ubuntuzilla project. This means they were only able to package it for 32bit since Mozilla does not provide 64bit binaries.