It's not worth me using this until Mozilla provides a new WebExtensions API's that makes it possible to display multiple pages simultaneously within the same browser window.
The reason for this is that it is clumsy to duplicate the menu bars, bookmark bars and toolbars. They become compressed at the top of each window giving a stunted interface on each instead of neat, open interface across the top. Tabs on the original window are hidden behind the windows tiled at the front.
This extension has some nice features and they will be great once they are applied to multiple pages displayed within the same window.