From MozillaZine Knowledge Base
To re-send a message that you've already sent:
- Select the message from your Sent folder (or other folder) that you want to re-send.
- Go the "Message" menu and select "Edit Message as New", or right-click on the message and choose "Edit as New". The message will now open in a new "Compose" window.
- Click the "Send" button on the toolbar.