We've shipped Betterbird 140.11.0esr-bb23 today. Please refer to the Release Notes for full details.
This release contains some significant improvements.
The story started when a user alerted us to a problem with reversed threading. We fixed that and while doing so, hit a few ancient threading issues described in this bug report. Miraculously, the Thunderbird folks woke up and fixed the issue and a few related ones, so threading is a whole lot better in Betterbird 140.11.0esr-bb23. Additionally, we fulfilled a long-standing wish to be able to collapse a thread to its most recent message and not the original message. This can be enabled with the preference mailnews.collapse_thread_to_most_recent. We dedicated this blog article to the subject.
One of two further issues we fixed by popular demand was better support for internationalized domain names (IDNs), also called Umlaut domains, since in Germany they are used to include äöü in the domain name. In general, that is not recommended, since even in the year 2026 it causes a host of issues. However, with Betterbird, sending mail to such domains works correctly now. Details in this blog article.
Finally we made resetting the folder sort order of subfolders available for all folders containing subfolders, not only server root folders. It's unclear why this restriction exists in Thunderbird.