Great add-on that definitely satisfies the niche after tab groups was removed by default. I like that it helps mitigate the problem of having too many windows open by allowing those tab instances to stay alive but remain hidden.

Feature suggestions.
I enjoy the option "Create a new group when opening a new window!" So, here are some suggestions for window options that may go along with it: "open a new group in a new (or the current) window defaults" to speed up workflow, and "delete (or ask for the name of) a(n unnamed) group when closing a window" to reduce the accumulation of unused groups when opening and closing windows. You could also replace this option with one to "delete groups with zero tabs" or otherwise keep track of groups that have been opened by New Window. Another option could be to "

*update* I use this almost exclusively in one window now and it works really well with Sidebars. Thanks for the excellent work!