You may have more than one version installed... Zorin comes with Thunderbird as an apt package by default.
Developers do not see the purpose of removing config files always. So some do, some don't.
Nothing should be under /snap, /flatpak, /etc/flatpak, but you still may have to remove ~/.config/[application], ~/desktop/[application].desktop, /usr/share/ applications/[application].desktop. there may be more, but I don't recall off the top of my head currently.