Double-clicking a message in the list opens it in full size or in a new window, depending on your message display preferences.
The header block now shows all message headers including a photo of the sender if one is found in your address book.
While looking at a message in the preview pane, it can be opened in a new window by pressing the button.
When opening a message in full-size view, the toolbar above shows buttons for possible actions to choose for this message.
Use the drop-down list on the top right of the toolbar to move the current message to another folder. When a folder is selected from that list, the message is moved there right away and the window will display the next message in the list.
Use the arrow button on the right side of the headers box to jump to the next or previous message on the folder. If there's no next or previous message, the buttons will be disabled.
Every email address referenced in the current message, either as sender or recipient, can be saved to the address book with a single click on the Add contact icon right next to it. Of course it won't be saved twice if a contact with that email address already exists in the address book.
If the opened message has any files attached, these are listed on the right, next to the message text. Click on the file name to open or download it. If the file can be displayed directly in the web browser, a preview window will be opened showing the attachment. This will happen for images, text files and PDFs if your browser indicates being able to display them.
Otherwise clicking the file name will initiate the download process and dialog asking you where to save it most likely appears. If that doesn't happen, check the default "Downloads" folder on your computer for the attachment file.
For files being opened in the browser directly, you can force download by either clicking on the Download link in the preview window or by right-clicking the attachment file name in the message view and then choosing "Save Link As..." from the browser's context menu.