The delivery format "auto-detect" does not work any longer on my desktop computer: I can still compose messages in HTML (set as the default), but they are sent only as plain text, although the delivery format "auto-detect" is used as usual. Only when I change the delivery format to "Send the message in both plain text and HTML", it works - but I now need to do that manually for each composed message - rather inconvenient. The problem is the same on all accounts. This behavior started on 2022-07-06 just after I installed a new S/MIME certificate, but on my notebook I did that as well without getting that problem. I tried various things which however all did not solve the problem. It seems to me that HTML composition works as normal, but at the end Thunderbird uses auto-detect and wrongly determines that it is not requiring HTML, and therefore sends out only plain text. That is even if I format some text, insert an image, have an HTML table in my signature, etc., which all clearly makes HTML necessary. The problem seems to be related to my Thunderbird profile folder. When I run it with an older version of the profile folder (about 6 months old), the problem is not there. But I don't simply want to switch to the old profile folder, since it might be tedious to get everything to the new state (messages, certificates, other things which I would need to research). Or could I copy only certain files of the old profile folder to the new one? Would it be something in prefs.js (which has many changes between those versions)? I am using the latest Thunderbird version (91.12.0 64 bit). My computer runs on Windows 10 Pro.