Hi and welcome.
First be sure to backup your thunderbird data. I am not sure how snap does this but a normal install will have it in a hidden file in /home .thunderbird. To view hidden files in your Home folder press Ctrl+ H.
Next, uninstall the snap package of Thunderbird. Open a Terminal and entter:
sudo snap remove thunderbird
Then remove the snap plugin to gnome-software:
Try this method of installation: