1. open the message
2. edit the URL from your browser's address bar. Replace the last slash (/) from the URL with ?compose=
For example, replace:
Gmail actually creates a new message with the same content and saves it as a draft. That's the reason why you can edit the message. When you no longer need the message, click the "close" button. You may need to delete the messages from the "drafts" label.
You can open multiple messages using the same trick. After using the instructions above:
3. open a new Gmail message
4. edit the URL from your browser's address bar. Replace ?compose= with & and the last slash (/) from the URL with ?compose=.