When you select an email and open it in tab or window and then close the tab or window, thunderbird should still be on the email you last opened because no other was selected. However, if you open an email in another window and then delete or move that email, thunderbird will auto close the window and auto select the next email which was above the deleted email; assuming you are viewing using sort order date, descending and unthreaded. So it will be of a more recent date. If the sort was threaded then the next selected email will be the one above, which may not necessarilly be the more recent date depending upon how the thread is contructed; it may contain subthreads.

If Thunderbird is behaving differently and you have not selected any keys to make it act on another instruction, then test running thunderbird in Safe Mode. Help > Restart with Addons Disabled.