So I deleted the Thunderbird 102 folder and copied the saved folder of the 91 version back. So it often happened that a mail, which was listed in the message list with subject, was shown as empty when opened. But already after one hour it was clear that I had to go back to the old Thunderbird version.

I would have accepted the defacement of the user interface. I then copied the complete folder of Thunderbird 91.x portable including the profile and tried an update. In the release notes it was said that an automatic upgrade from the 91 developer version to the 102 branch is not possible for the time being. But there was nothing vital in my eyes, which would have made an immediate switch necessary. The list of new features was quite extensive according to the release notes. Then, at the end of June 2022, the developers released version 102 (see Thunderbird 102.0 and 91.11.0).

Therefore, new development branches arrive at users in regular succession. The development of the email client Thunderbird is somewhat aligned with the development cycles of Firefox.

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