Currently, while using the toolbar on the left to switch between mail, calender and contacts, Thunderbird opens a new tab. Even if you switch back to mail, the other tabs remain open.
There should be an option to disable the tabs and only switch using the toolbar on the left.
Why: opening tabs is a behaviour expected from a web browser, not an e-mail client. Thunderbird offers a dedicated toolbar, there is no need to open tabs.
Benefits: gain of space, more intuitive for new users