Threaded view missing replies
It appears that Thunderbird 52 has broken Thread viewing as the replies to the message no longer show up when viewing messages in the Inbox. All one sees is the inbox messages none of the replies to those messages show up.
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Sigh it seems that 52 has broken several extensions one of them was Conversations which was allowed me to see threading emails including my replies interspersed in with the received emails.
If you have an issue with an addon then you need to inform the author of the addon as it is their job to make sure their addon products continue to work with Thunderbird. It is not the job of Thunderbird to work with addons, but the other way around.
Conversations Addon: https://addons.mozilla.org/en-GB/thunderbird/addon/gmail-conversation-view/
There is a section discussing version compatibility.