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Yes, this is an Ubuntu Thing. With Ubuntu 24.04 (the Base of Zorin 18) Thunderbird comes only as Snap - like with Firefox in Ubuntu 22.04. It is the same Thing.
Your Options:
-
Use the Flatpak:
https://flathub.org/en/apps/org.mozilla.Thunderbird -
Download Thunderbird from the Website:
https://www.thunderbird.net/en-US/thunderbird/all/ -
Add the mozillateam PPA from Ubuntu:
Firefox ESR and Thunderbird stable builds : “Mozilla Team” team