No responses! Is it a difficult problem or no one else has faced it?

There is a preference that you can try, but I've detected something not correct in my version which is not a portable app version.

• Menu app icon > Options
• Select 'General' in left pane
• Scroll down to bottom - it goes on forever :)
• Click on Config Editor button
• Accept the warning
• In search type: mail.tabs
• Look for this line : mail.tabs.loadInBackground
• Double click on line to toggle the value and set as 'True'
• close about:config window top right x

Restart Thunderbird to ensure new preference is stored correctly.

Thanks. When I checked this option was already set to true. So unfortunately, this did not work. I have also noticed that if I open the mail in new tab by using the context menu, the mail opens in the background. Weird.

Yes, I've noticed the preference setting seems to control the focus when using the drop down menu, but not the double click to open in new tab.

I've created a bug report to see what developers think. This is a link to it, if you want to see future comments. https://bugzilla.mozilla.org/show_bug.cgi?id=1667595

I've just seen the coding behind all this and this is what is set up. The default setting as true means if you right click and select open message in tab, focus does remain in thread pane. A double Middle click on mouse opens in new tab and retains focus in thread pane. (assuming user has a mouse that provides this) But a normal double click opens in tab and does have focus.

This provides the user with a symultaneous means of choosing whether to open in background or with focus without needing to swap the preference setting.

Unfortunately, I do not have a middle click on my laptop mouse pad. For me double clicking and retaining the focus in the message pane is a great productivity feature. Though this behavior seems to be by design, not all laptops have middle double click.