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 ....