re :An automatic update to v 91 just completed Thunderbird is not updating from 78* to 91* If you had version 78.12.0 then a Thunderbird update would update to version 78.13.0 If you have another updater software on your computer performing the update then it would not be aware that Thunderbird is currently not auto updating. This info is available in release notes: Thunderbird 91 is only offered as direct download from thunderbird.net and not as an upgrade from Thunderbird version 78 or earlier. https://www.thunderbird.net/en-US/thunderbird/91.0.1/releasenotes/ Some people are not even aware that another program either updater software or even some Anti-Virus products have that ability to say another version is available and even force the upgrade. re : Calendar There are many users reporting the calendar is missing after the update. If you were using the Provider add-on, disable or remove it, then add the calendar directly as a CalDAV calendar: * https://support.mozilla.org/en-US/questions/1304356#answer-1368791 * https://github.com/kewisch/gdata-provider/issues/194