rer :Now, the new profile has these quirky issues where it won't open the inbox in a window showing the header information that allows one click to open the message immediately below.

Single click means display in the 'Message Pane' IF the Message Pane has been enabled.

Have you enabled the Message Pane ? Menu app icon (3 lines) > View > Layout > select 'Message Pane'

re :Further, the new TB client is constantly downloading and updating folders with old emails from the server

If you created a Pop mail account then it would download whatever was in the server Inbox and put it in the Pop account Inbox.

If you created an Imap account then that means it will synchronise all subscribed folders to display exactly what you have on the server in the same folders. Imap accounts can only display whatever is on server. The server view as seen in webmail will be the same as seen in an imap account and if you have more than one computer set up to view same account and it's set up as imap then all will synchronise with server to display identical views.

If there are a lot of emails that need to get downloaded then they may download in batches as there may be download restrictions on volume or size limits set by server.

re :and of course, it adds a few more emails to the 'Trash' file each time.

If emails are in Trash when viewed via webmail account, then they are going to be in Trash in the Imap account.

If you have any 'Message Filters' I suggest to check to see if any of them have an action which puts emails in Trash.

If you have other computers with accounts to same email address then if you move anything to Trash in those accounts and it updates the server then the imap account will also appear to put those emails in Trash. That also includes if you are using a phone to see emails and you choose to delete some.

It can also occur if you have settings to delete emails that are older than a specified date. Account Settings > Synchronisation & Storage (for imap) OR 'Disc Space' (for pop)

Do you have you 'Junk Settings' set up to move to Junk/Spam or set up to delete in which case they will appear in the 'TRash.

Thanks for the info on enabling the message pane which as it turns out is the 'classic' view I've always had. (Had I tried to clone the Prefs.js from the original client profile I would imagine the classic message pane would have returned on its own. It isn't even a setting I'm familiar with, or is it a hold over from days gone by?

I may not have made this clear, but I have 5 email profiles that are IMAP as the server I'm on does not support POP anymore.

Until just two weeks ago all five were identical, worked perfectly and no changes had been made to preferences. None had ever begun examining email folders or begun deleting emails. One email profile began this strange behaviour and it, of all 5 is the oldest profile I have.

It developed one new issue with the message pane once re-created, but still had this weird behaviour continuing to sift through folders and dump emails in the 'Trash' bin. Four days ago I watched as it went through every folder, before I removed the account and files. None of the other profiles have this issue, and all have the same IMAP protocol and retention settings. Four of them can only be accessed via TB and this problem one can both be accessed by TB and iPhone.

(And this raises the question as to what impact the iPhone may have on this client profile??) Interestingly, there was an iOS update a day or so before this client profile began misbehaving. Coincidence?

Fortunately, as you mentioned, the client can only store and display what the server has, and this may explain why, after deleting emails it subsequently downloads them yet again. At present there are over 1772 deleted emails in the 'Trash' bin, yet the same emails are still in their respective folders.