I can't help with the Beta version as I don't run it. But there is a difference between the different search windows in Thunderbird and what they search in. Details are here: https://support.mozilla.org/en-US/kb/quick-filter-toolbar https://support.mozilla.org/en-US/kb/global-search Not sure if you're using the incorrect search window but these articles should help.

Thank you frisee but these links are of no help to me. They do not touch on searching messages in subfolders.

Have you tried with Classic Search (Ctrl+Search+F) and enabled 'search subfolders'?

If you're using Global Search (Ctrl+K), make sure GS is enabled in folder Properties (context menu).

Finally, rebuild the global database:

https://support.mozilla.org/en-US/kb/rebuilding-global-database

Thank you. Subfolders search is enabled in the Classic search. But it restricts me to search on a single folder on a single email address. Which is not at all convenient for me. I have ten email addresses with over 200 folders and subfolders.

I am hesitant to rebuild the global database because as things stand, the app is ever downloading thousands of emails even when idle.