Updated Review: The recent update a day ago resulted in the add-in being unable to load. Agreed, I have a lot of tabs open (approaching 90), but I have plenty of memory and that's why I use this Add-in.
In fact, its why I use FireFox.

Reinstalled v 3.9.22
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Been using this for 10 years. My 5 * review from 7 years ago is below.

Please continue supporting this for FF57. It shows up in my FF56 still as being a legacy extension. Like lot of others, it's the reason I use FF. I like being able to drag a group off-pane and creating a new FF window with these tabs present.

My main suggestion for improvement though would be for the new Tabs to appear at the bottom of the list, not as children of a current tab. Swap the current Cntl Click function to creating tabs as child tabs.

It would also be nice if the currently active Tab changed colour to something bright and in yer face. I'm often on screens and lose track of which tab it is from when I wish to close the screen.