I like the basic concept but what I definitiely do miss (based on my experience with other tab group add-ons) is an option to hide the currently not used tabs and tab groups from the browser's tab bar. I even think that such option should be activated by default (to avoid tab proliferation in the tab bar).

Use case example: When I'm in the office, I often have some "not direclty work-related" tabs opened (as probably all of us do). And I do not necessarily want my colleagues to see the titles of those tabs opened in a related group which I call "private". When a colleague steps in and we are working together on something using my screen, I want to show only tabs from the active group which is usually work-related (and have the tabs in the "private" group hidden from the tab bar).

I guess this could be easily achieved by adding an additonal option in the settings "Only show active group in tab bar" or so, where "active group" would be the one selected in the side bar.