I realized that the solution to my problem was as simple as turning threading back on, which is off by default (apparently Thunderbird reverts to default settings after some updates).

How can I turn threading back on?Can't find at menus.

Try this: https://bugzilla.mozilla.org/show_bug.cgi?id=1393520

To turn threading back on in a folder: View - Sort by - Threaded I am not sure how to turn it on universally for all folders, which would be helpful.

Please see attached file "open in conversation" is deactive.I could not find how to activate it.

Try following instructions in the link above.

I am running 60.4.0 (32bit) and I have seen similar behavior. I have used the "open message in conversation" menu item many times a day for years. Since upgrading to this version (from the an older major revision). I have noticed a bit more detail on the issue. When ever a new message comes in, that is in a conversation, it will not have the menu item available. But... after something, usually requiring time, it will become available. I have done a compact, and seen it appear. I have opened in safe mode and seen it appear. I have waited some time and seen it appear. It is as if new messages are in a cached state, and eventually get truly processed?

Sorry - previous post should have said - I have noticed this issue since upgrading to this version (from the an older major revision).

I have just seen that a few messages did not have the context menu, I shutdown Thunderbird and reopened it, and they all do now. I really think there is some delayed processing that is causing this. Unfortunately, my work flow generally involves using this soon after new mail arrives. I have been trying to find some action I can do (short of restarting Thunderbird that will trigger the processes, but no luck yet. It seems that compacting might in some cases, but certainly not always.

"Global Search and Indexer" might also be the cause. See this [https://support.mozilla.org/en-US/que.../1255781] for more information.