Perhaps you could draw on a piece of paper how that treading would work. Given the older message must be further up the tree than the new one so they can be attached to one another.

Or

Perhaps in a previous version you used the conversations add-on that mimicks googles conversations. which are not threading of messages at all.

I'll type an example:

Desired thread sorting (ascending):

(1) Original (oldest) message

    (2) Reply to #1 message
         (3) Reply to #2 message

This is how TB displays them with my version (descending):

(3) Reply to #2 message

    (2) Reply to #1 message
         (1) Original (oldest) message

Gmail's conversations, to my knowledge, also use a descending order, which is the opposite of what I'm after.

I simply did not think that was possible.

View menu (alt+V) > sort by.

Set Date Set Descending Set Threaded

Thanks, Matt, but the method you described will sort the messages in descending or ascending order. But this will not change the ascending/descending order of the threads within the message. Hope that's explained clearly.

My end goal here is to have a conversation read like, well, a conversation. Like you'd read a thread in this forum, or a conversation out of a book. It's odd that email threads go the opposite way.

A "thread" means a sequence of messages. I've never worked with an email client that would edit messages to rearrange quoted messages within a message. You took care to emphasize "within the message". I don't think doing that is feasible.

In the past in multiple versions of Thunderbird replies from a single post were always sorted without any adjustments from me with the most recent at the top.

As Grunzer above, I'm not talking about the messages themselves. I'm talking about the replies within a message.

How do I make the most recent appear at the top?

Thank you.