At the end of August 2019 the ESR version 68 (ESR stands for Extended Support Release) of the popular mail client Mozilla Thunderbird was released. The release is a continuation of a development that started with Thunderbird 60. The user interface now comes with the settings moved into a tab (as seen before in Firefox). There is also “dark theme” for those who want to read their mail white on black.
There’s more going on under the hood. The switch from XUL add-ons to the new WebExtensions – now also called MailExtensions on the developer pages -, is going on, unfortunaltely causing some add-ons to be deactivated during the upgrade because they do not yet use the new APIs.
Users must therefore check for themselves whether a compatible version of the extension exists, that may be installed from the extension directory accessible via the add-on menu entry. If the respective developer did not provide for an upgrade, the user is left with nothing.
That’s basically why Ubuntu did not ship the upgrade directly, but waited until October 2019, when Ubuntu 19.10 was released. One Ubuntu maintainer explicitly mentioned problems with the jsunit and Enigmail packages as the reason for the delay. For the LTS releases 16.04, 18.04 and 18.10, which are also still supported, the upgrade took even longer; they remained on version 60 even after the release of Ubuntu 19.10.
See the catalog entry for Thunderbird