Cant' see how to edit my question so adding this to clarify.

'Check for new messages at startup' is currently unchecked. It matters not whether this is checked or not. The expansion of folders happens on first check for new mail. So, in my setup, after 3 minutes, the account folders expand!

The Thunderbird developers appreciate people who are willing to run the beta as this helps to test the software to locate any bugs etc before it gets to the release version.

The latest release is version 91.7.0. https://www.thunderbird.net/en-US/thunderbird/all/

The Folder Pane should display the same view upon startup as it was left in when you exited Thunderbird.

After exiting Thunderbird, it is advisable that you allow a little time for background processes to complete before shutting down computer. This allows session data to be stored and/or files to be updated.

If you have any software on your computer which can clean up files eg: CCLeaner, WiseCleaner or even some Anti-Virus software can clean up files - it is advisable to make your Thunderbird profile folders exempt from the 'clean up' as those programs can remove eg: session data.

Do you allow your Anti-Virus to scan folders and files upon opening - suggest you make Thunderbird profile folder exempt?

Check to see if there is possible corruption with the folderTree.json, session.json and the xulstore.json files:

In Thunderbird Help > More Troubleshooting Information Under 'Application Basics' section - Profile Folder - click on 'Open Folder' A new window opens showing contents of profile name folder.

Exit Thunderbird now

scroll down - Locate and delete these files:

• folderTree.json
• session.json
• xulstore.json files

Start Thunderbird

Set up folders as required in various collapsed and expanded state. Make a note of what is selected - perhaps a screen dump may help. Use for a short while and then exit Thunderbird Wait a short while and restart Thunderbird.

The Folder Pane should display the same view upon startup as it was left in when you exited Thunderbird.

If I leave the 'Check for new messages at startup' unchecked, it does startup the same, until the initial mail check, which depends on the 'check for new mail' interval, at which point all folders whos mail is checked expand.

If you have any software on your computer which can clean up files eg: CCLeaner, WiseCleaner or even some Anti-Virus software can clean up files - it is advisable to make your Thunderbird profile folders exempt from the 'clean up' as those programs can remove eg: session data.

I don't use any of that crapware.

Do you allow your Anti-Virus to scan folders and files upon opening

No.

Check to see if there is possible corruption with the folderTree.json, session.json and the xulstore.json files: In Thunderbird Help > More Troubleshooting Information Under 'Application Basics' section - Profile Folder - click on 'Open Folder' A new window opens showing contents of profile name folder. Exit Thunderbird now scroll down - Locate and delete these files:

• folderTree.json
• session.json
• xulstore.json files

Start Thunderbird Set up folders as required in various collapsed and expanded state. Make a note of what is selected - perhaps a screen dump may help. Use for a short while and then exit Thunderbird Wait a short while and restart Thunderbird.

Ok, did that, no joy.

Actually, as soon as I posted this question the issue appeared to go away. I stopped and started Tbird a number of times without issue. I then followed your suggestions and still all good. However, this morning it's up to it's old tricks.

I'll maybe revert back till I get to the release where the problem started and then turn off updates!

Thanks for the suggestions.