Updating Thunderbird 78 to 91 on Ubuntu 20.04

The newer version you installed is a Snap package. The old version is coming from the regular Ubuntu software sources.

You can remove any of these using the Snap store. You will find two versions there. The one coming from Snap.io is the Snap version. Remove the other version.

The Thunderbird profile of your current version is stored under ~/.thunderbird. The profile of the snap version, however, is stored in a .thunderbird folder under ~/snap/thunderbird/common.

To have your current profile settings adopted by the snap version so your email accounts and settings are preserved, first make sure none of the Thunderbird versions is running. Then delete (or rename) the directory ~/snap/thunderbird/common/.thunderbird and move the ~/.thunderbird directory to ~/snap/thunderbird/common/.