There are repeated requests in the forums for adding a close button to the layer/tab buttons. Firefox and Opera users might be used to it. Our current middle/double click method is not self-explaining, not all mice have a middle button and a double click is less convenient than a single click.
Such a button should be optional. Other users may consider it as a waste of screen space.
A space-saving alternative might be to use the website icon for this purpose (replace by a 'close' icon when hovered).