Every browser listed supports this onunload attribute. However, it should be avoided as it encourages the mixing of content and behavior.
In fact few browsers these days accept mailto as a form action. You need a really old browser for it to work as more modern browsers simply open the email program (if any) and ignore the form. If you are going to use a form then use a server-side formmail script as the action - there are several good free ones out there.