About this Add-on
To use this add-on, just reply to your messages as usual. When the composition window opens, this add-on will add a button labeled "Send and Archive" to the toolbar allowing you to send your message and archive the original in one step.
If you're not forwarding or replying to a message, the "Send and Archive" button will hide itself to help free up space on your toolbar.
New in Version 2.0: You can archive the whole original thread by going to Send and Archive's preferences (Add-ons -> Extensions -> Send and Archive -> Preferences) and checking "Archive whole thread".