G.Harvey said

I recently changed my Thunderbird to IMAP rather than POP by adding my email as a new account and deleting the old one. Has this removed some kind of profile setting? Before I had always had cookies set to "Keep until I close Thunderbird" and would like to continue the same. But with that setting, every time I close Thunderbird it forgets that I have set Message Pane to OFF. If I change cookies to "Keep until they expire" then I can close and reopen Thunderbird happily without the Message Pane appearing. I notice that there is a "thunderbird.net" cookie. This seems to be what is remembering my Message Pane setting. I will try rebooting with that cookie in place and see if it helps.

But it did not help - Thunderbird opened after rebooting my laptop with the message pane displayed. "thunderbird.net" cookie was a red herring - presumably it is just the welcome to Thunderbird message that arrives with each new version. I have deleted it now. Perhaps best just to live with the irritation of having to switch the message pane off, every time I open Thunderbird...

Hi This won't answer the original question ... but assuming you don't already know this, it may help your daily irritation. F8 is a quick way to toggle the preview on and off. Hope this helps, Agnes

I don't think cookies or POP vs. IMAP have any effect on the Message Pane, but if its status is not being remembered between restarts, you might be able to fix it by opening the profile folder (Help/Troubleshooting Information, click Open Folder), closing TB, and deleting or renaming session.json.

CCleaner, if you use it, is another source of erasing TB settings.

Thanks for the suggestion. Tried that a couple of times but no effect. There is also another 'JSON' file (whatever that is), called "sessionCheckpoints.JSON". Would it be good to delete that too? I don't use CCleaner. The problem occurs between closing and re-opening TB with out doing anything else in between.

The sessionCheckpoints.json file doesn't seem to contain anything to do with the Message Pane (view json files in text editor), but you could rename it and see if it helps. Also, try running TB in safe mode (hold Shift when you launch TB) and see if there's any difference, to test add-on or layout issues.

Thanks for not giving up on this problem! All is now well. Apologies, but I cannot give a clear account of what happened. I did not include a reboot when I followed your first suggestion (deleting the session.json file), but I shut the laptop down last night and when I rebooted this morning, the message pane was satisfactorily absent. Hurrah! Something has changed behind the scenes....Thanks for your help.