I found the new tab system in Thunderbird 3 highly disruptive, because I would generally skim through an email and attempt to close it by clicking on the top right corner – only to find that I had closed the entire Thunderbird application instead.
Worse, when it opened again next time, it now displayed the last email I had read – open and ready in a tab – mockingly.
Unlike in web browsing one rarely has to keep multiple emails open, so this tab idea for Thunderbird may not actually be a good one.
However, there is a way to remove the tabs. Go Tools>Options (on Windows, or Thunderbird > Preferences on Mac) click on ‘Advanced’, then on ‘Reading & Display’ and select ‘A new message window’ as the option for ‘Open message in:’.
After doing this, if the solitary single tab that is left annoys you, you can auto-hide it thus:
Under Options>Advanced, click on ‘Config Editor’ button (which is under the ‘General’ tab, and Search for ‘mail.tabs.autoHide’.
Double-click on it so that it becomes ‘true’. Close the window.