Are there reasons that the "Simple Tab Groups" extension isn't suitable for you? It seems to be well-maintained, cleverly uses Firefox's "hide tabs" API to keep a single window behind-the-scenes, and has integration with lots of other features like Firefox's containers. I think it's an indirect descendent of the old Tab Groups extension when it was spun out of Firefox.
It doesn't scale. Also we intuitively identify a tab by its location in our browser. Opening a different window and then locating that tab there is slow.
https://github.com/drive4ik/simple-tab-groups