This works: - exit thunderbird - in profile, copy prefs.js elsewhere in case of problem - open prefs.js in text editor, such as notepad - insert this line before first user_pref line

  user_pref("mail.pane_config.dynamic", 0);

- save and restart thunderbird

NOTE: if there are multiple prefs.js, e.g., prefs-1.js, that indicates something is preventing thunderbird from saving settings. If problems surface, exit thunderbird and copy the backup prefs.js back. I did test this prior to posting.

If that is the solution then why do Thunderbird’s developers not add a mail.pane_config.dynamic option to the Config Editor so that it may be set to 0? That is much simpler than editing the prefs.js file.

What I gave you was a workaround. The developers did a super job of making it easy: just click View>layout>classic view. Unfortunately, something is keeping thunderbird from updating the prefs.js by itself.

Unless the prefs.js file has that crucial line, it will be just as easy for Thunderbird to revert to Vertical View as it is for its user to change the layout to Classic View. This is a serious bug in Thunderbird which I plan to report and hope the developers resolve ASAP.

Close TB, open the profile folder, delete or rename session.json, restart TB, then see if the layout setting is retained between TB restarts.