This link may help: https://support.mozilla.org/en-US/kb/message-threading-thunderbird and there is an addon, 'thunderbird conversations' that you may wish to pursue.

Thank you, I'll check it out and come back to mark this as solved if applicable.

re :2. The DESCENDING view does this, which I feel is confusing: a) Single messages and thread topics in the selected folder are sorted descending. For threads, it weirdly uses the timestamp of the FIRST message in the thread instead of the most recent message. b) Messages within a thread are sorted ascending instead of descending.

That is because the 'Sort by' only applies to top level emails, so if you use 'unthreaded' then all emails are single top level emails and all emails will get the 'Sort by' option applied. If you use 'Threaded', then the 'Sort by' gets applied to the initial email and not it's sub threads/replies. Those emails are treated as a conversation, so each email is appended to the initial email meaning the newest will always get appended to the bottom. If you have ever used eg: Facebook messaging chat then you will notice new comments are below the previous / appended to the conversation.

This issue has often been asked int he Bugzilla forum and there seems to be no interest in altering the current set up. However, if you choose to right click on initial email in thread and select 'Open' > 'Open Message in conversation' then the entire thread is opened in a new tab. You can then sort by date if required. It means it's much easier and less cluttered to just see everything to do with that conversation including any replies you have made. Try it out :)

Helpful information, thank you for the tip, and thank you both for your replies. It is surprising to me that there seems to be no interest in making this work like Outlook, as that feels like the most 'natural' way to deal with threaded messages. I'll mark this as closed.

Indeed, super helpful information!

re :2. The DESCENDING view does this, which I feel is confusing: b) Messages within a thread are sorted ascending instead of descending. ... If you use 'Threaded', then the 'Sort by' gets applied to the initial email and not it's sub threads/replies. Those emails are treated as a conversation, so each email is appended to the initial email meaning the newest will always get appended to the bottom. ...

Thanks, @Toad-Hall for the thorough explanation, but I'm still not sure I fully understand: So, when you say the threaded messages within a thread are treated as conversation, let's say when people reply to an email that is not the last email in the conversation, does that mean they sort of start their own "conversation" and any email replies to them remain grouped together? Essentially "as if" there was a kind of Reddit-like tree view (only that it's not visible)? I couldn't make sense otherwise of the order of the emails within each thread.

It would be so much easier if the developers included the old method of using a physical line on the left side to show how the conversation developes.

If it's only between eg: two people then generally there is a to and fro, one follows the other. So newest will always be at the bottom. But there are also occasions when more people are included and it's possible someone replys to the original messages and someone else replies to one of comments in the thread. The appended comments will be indented when it's a straight forward reply to the one above and dates will always be in ascending order, but then you might find another is outdented as it is a reply to an earlier comment. In which case it may also have an earlier date. So, a reply email is always appended to the relevant email it was in reply to and always in date ascending so newest is always last in that part of conversation. These 'threads' are based on people specifically 'Replying'. A new email with a new subject would not be part of conersation- although you can never account for someone messing things up by Replying and altering the Subject just because they were too lazy to enter an email address, when in reality they should of started a new email and not a reply.

Example: Person 'A' - Initial email to several people. date: 01/01/2025 - Sort by Date Descending is on this Email |> Reply by 'B' to original date: 01/01/2025 > |> Reply by A to above reply by B date: 02/01/2025 > |> Reply by C to above reply by B date: 03/01/2025 > > |> Reply by B to above reply by C date: 05/01/2025 |> Reply by C to original date: 02/01/2025 |> Reply by D to original date: 04/01/2025 Person 'A' - Initial email to several people. date: 25/12/2024 - Sort by Date Descending is on this Email