Thanks Firefox for killing this great app...

The classic version was a great Firefox add-on which I used on a daily basis. Displaying different pages within the same Firefox window in the layout you wanted. Now every page contains a full Firefox window which is a waste of space!
Even more annoying is the fact that the new windows open on your primary screen, which is not necessarily the window where you have the original Firefox session open. (If you work with multiple displays)
This is not a developer flaw but because of the limitations of web-extensions and Firefox 57 removing the technology they used in the classic version.

5 stars because the developers did an amazing job on the classic version. Hope you find a ingenious idea to make it perfect again