I had been holding off updating to TB 78.5 from TB 68 due to issues 78 breaking extensions. Thunderbird finally pushed the update upon me and then discovered one of the extensions Thunderbird Conversations which enables one to view a thread in conversation view in Thunderbird '''appeared''' to be misbehaving. I started this [https://github.com/thunderbird-conversations/thunderbird-conversations/issues/1564#issuecomment-744012512 thread] over at Thunderbird-convesation plugins outlines in detail the issue I am having. The developer just recently asked me to * Select one of the messages * In the three-dots at the right of the message header in conversations, select "view using the classic reader" * On the classic reader display, select the more button at the end of the header, and select "Open In Conversation" Which I did and it appears that the culprit, in this case, is Thunderbird 78.5 which is grouping distinct unrelated thread together for some unexplained reason.