Thunderbird desktop app has Tuta Calendar and Tuta Mail web-app integration via add-ons

Tuta

Tuta Mail and Tuta Calendar now available as Thunderbird add-ons | Tuta

Good news for all Thunderbird users: Now you can use Tuta in Thunderbird with just one click!

Due to built-in end-to-end encryption, Tuta Mail and Tuta Calendar cannot be integrated into Thunderbird via IMAP or CalDAV. However, we now have a workaround for all Thunderbird fans: Tuta Mail and Tuta Calendar are now available as separate Thunderbird desktop add-ons. You can easily access the Tuta web app via handy quick access buttons in the Spaces Toolbar. And with just one click, Tuta opens directly in Thunderbird, each in a new tab.

Technically, this is secure web app access within the Thunderbird desktop app—without any restrictions on functionality.