For installation mozilla thunderbird 3.0 mail client on Ubuntu you need to do following steps
Add mozilla daily repository to you sources list
gksudo gedit /etc/apt/sources.list
#and add following lines
deb http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu karmic main
deb-src http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu karmic main
If you use another dist of ubuntu, change karmic to jaunty, hardy, karmic and intrepid