Description of Problem:
When an email is opened, and you click on one of
the buttons that produces a new email (Reply,
Reply-All or Forward), the original window should
be closed as the new compose window is opened.

Steps to reproduce the problem:
1.  Open an email
2.  Click "Reply" (or "Reply All" or "Forward")
3.  Compose and send this new email message

Actual Results:
Original email window is still open in background

Expected Results:
Original email windows should close (optional)

How often does this happen? 
Always

Additional Information:
I searched Bugzilla and didn't find an existing
bug, although I'm sure others have asked for this
basic feature (I'm kind of surprised that you've
made it to 2.0 without it).  So please assign as
dupe of the existing bug that I couldn't find and
I'll track that one :)