One of the most useful features of Thunderbird is the ability to open messages in a unified conversation view, irrespective of the folders in which the constituent messages are stored.

Recently, this stopped working on one of my machines, for messages in my inbox folder.

I tried rebuilding the message index - closing Thunderbird, and deleting global-messages-db.sqlite from my Thunderbird profile directory - but that did not work.

“Repair folder” also did nothing.

What fixed it was right-clicking on my inbox in the folder pane, selecting “Properties”, and ticking the box “Include messages in this folder in Global Search results”.

I don’t know why this become unticked, but there we go.

I have conversation view back again.

If it happens again, I should see if there is an option to include all folders in Global Search results, rather than doing it on a per-folder basis.