Open in Conversation no longer an option in Thunderbird 128.5.2esr.

Are you sure? In my Mac version of TB 128 this is still possible. Via "Customize" you can drag the button "Conversation" to the tool bar and R-click (on a message) menu allows to select Open > "Open Message in Conversation" (see screen shot below)

Thanks but it is not working for me.

R-click on the folder in question > Properties > General Information .... Check "Include messages in this folder in Global search results"

Thank you but it is already checked.

I appreciate your trying to help. Just to clarify, I am an IT engineer, this folder, and in fact all of my folders, are included in the Global Search results, and I have already rebuilt the index.

What I had not tried was to rebuild my inbox. That has worked!

Good find! As a matter of fact: when I "uncheck" the option "Include messages in this folder in Global search results" the item "Open Message in Conversation" disappears from the context menu. But I'm only a physician, not an IT engineer ....

I am having the same problem. To be honest this is a constant in the last years, I would say, maybe longer. Sometimes it works, sometimes it does not. Deleting the database global-messages-db.sqlite does nothing. It is the first thing I usually do when this start to happens, but I do not remember any single time that this action worked, to be honest.

And you will ask me, when does it start working? The sqlite database is rebuilt in one day, but that does nothing. I have not a clue why it starts to work. I just wait, usually a couple of weeks, a couple of months or a year and one day starts working again.

Meanwhile, I just wait.

What I had not tried was to rebuild my inbox. That has worked!

Sorry, I spoke too soon, and then forgot to update the question. No, the above does not work; I got fooled by the fact that when you close Thunderbird and re-open it, all the emails present in the inbox at the time of re-opening will then work with shift-control-O, but any new incoming emails, regardless of how many messages are in the conversation, will not.

Is anyone working on this?