This appears to be IE 9 specific. I could not reproduce in Firefox. And found it hard to reproduce in IE9. This jira is therefore for documentation purposes in case the issue emerges in production later.
In Messages setting
You have Auto Forward Messages
Clicking radio button from yes to no
And focus and add a character in the E-mail address for forwarding before it becomes read only freezes IE 9 totally for about 5 minutes until it unfreezes itself.