I tried to used these commands at the root terminal:
sudo sed -i -e 's/archive.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list sudo apt-get update && sudo apt-get dist-upgrade
The result is it could not find.
Then using the add/ remove GUI, I also could not load the Thunderbird to install.
I have very little Linux command text knowledge. How can I get and install Thunderbird into my old Ubuntu V9.xx?