Thunderbird 91 was launched two months ago, but it wasn’t offered as an OTA (Over-the-Air) upgrade from Thunderbird 78 and earlier releases. After three minor point releases, Thunderbird 91.2 is here and enables support for OTA upgrades from earlier Thunderbird versions.
Thunderbird 91.2 is also a minor update, bringing only the ability to use a unique filename when saving a single message in the .eml file format, as well as a bunch of bug fixes. But the major change in this release is the ability to upgrade from Thunderbird 78 and earlier versions without from within the app.