The Thunderbird email client installation package is part of the official Ubuntu package repository and is installed as a default email client for Gnome Ubuntu 18.04 desktop. In case the Thunderbird email client is not already installed on your Ubuntu 18.04 you can use the apt command to perform the installation.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-install-thunderbird-on-ubuntu-18-04-bionic-beaver-linux