I'm using Windows OS Thunderbird windows32bit version 68.2.2

Test: Set up to auto open in a new window open email in a new window Select 'Go' > 'Next' > 'Message' Shows next message in same window. Select 'Go' > 'Previous' > 'Message' Shows previous in same window

Short cut keys 'Alt' > G then N then M all ok Also check shortcut N to get to next unread etc and all is ok.

Then use the 'Customise' option to put 'Next' and 'Previous' icons on toolbar. checked both and all is working as expected.

What OS are you using? What version of Thunderbird?

Test in Thunderbird Safe Mode to see there is any addon etc effecting the results and report back on results. https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

I added the previous/next arrows to the toolbar but they are grayed out and do not work.

What happens in Thunderbird SAfe Mode ?

Thanks for your response and I apologize for not getting back to you.

I am on "Windows 10 Home", version 1903, OS build 18362.720. Thunderbird 68.6.0

My test:

   I confirmed that I am in Safe Mode by looking at Tools->Add-ons
   In Options/Display, Open messages in = A new message window.
   I opened a message in a separate window.
   If I use Go->Next->message in the main window, the highlight moves to the next message.
   If I do the same in the message window, the highlight in the main window does not move.

After rereading the issue, I confirm the same. Maybe I misunderstood the first time as the requested GO Next email was displayed in the second window.

This is (I expect) because you are using two windows that act independently. This is also the case if you read in a separate tab. The operation occurs within the window or tab to display Next / Previous etc. It is GO Next on the display of email in that window or tab and not on the Thread Pane.

for example, I can remove the 'Menu Bar' toolbar in the opened email window, but that does not mean I have removed the 'Menu Bar' toolbar from the main Thunderbird window.

When you only select email in list so message displays in lower Message Pane, the Go Next operation is on that list. In order to display in Message Pane, the email in Thread Pane must be selected.

I take this to mean that you do not consider this a bug. I that case, can this complaint be converted to a feature request?

You can request an enhancement/post bug report via bugzilla. You would need to register in order to post a request for enhancement.

https://bugzilla.mozilla.org/home Select the 'New Bug' option and 'Thunderbird' Type a few words as title to see if there is another reported. It is possible you would not locate an issue exactly the same. When entering info about issue, mention what version you are using and your OS. Be clear instating the steps and results.