You will have to download thunderbird-dbg from the repository you linked to BEFORE you can install Thunderbird itself. You may find there are other things you'll need to track down too.
A better bet would be to get it from https://archive.mozilla.org/pub/thunderbird/releases/91.1.2/linux-x86_64/. Choose the language you want and click that then thunderbird-91.1.2.tar.bz2. That is a tar achieve file so when it's downloaded open it and you'll find a folder called thunderbird. Copy that wherever you like. To run it either open that folder with your file manager and click the file called thunderbird. You can also add a custom launcher for it to your menu and/or panel.
