Special:EmailUser should ask the user's confirmation if the user wants to close the tab. It could be very frustrating when you accidentally closed the tab and cannot restore it.
No, please not.
Another solution is that you write a user gadget, or that you ask the people who write your web browser to implement restoring values when you reopen a recently closed tab.
"Are you really sure you wanted to click what you just clicked" dialogs are annoying as hell. An entire generation’s computing experience is built around clicking ‘OK’ to dialogs that they either don’t read or don't understand.