This is really cool! I would say though that the auto-created groups move themselves to the far left when a tab is added, overriding the manual placement of a manual group.
I'm mainly a Firefox user on desktop and I don't think I'd use it there, since Firefox already has great support for containers and tab groups (Simple Tab Groups and Multi-Account Containers for me).
However, I will say that from the video demo, your extension looks really slick. It doesn't quite work in Edge (some crashes, collapse/expand doesn't work right), but I set it up in my Chromium install so I don't forget about it.