I've installed the add-on just yesterday and already enjoy having a tab group per project instead of an unsorted pile of tabs. It works really good so far.

Just one problem I had: when I created the first group and switched into it, I was unable to find a way to go back to the ungrouped tabs. Is there even any? I think there should be an explanation in the readme.

I'm glad there was a simple solution: the app offered to move the tabs into the current or a new group, so I just put the ungrouped ones in a group called "Default" for now.