"Tab" in the name here is misleading as this add-on isn't about tabs at all. It just deals with bookmarks per se. You know, like in the Bookmarks menu. Just without any reasonably expected functions for dealing with bookmarks.

This should actually be called "Browser Bar Bookmark List" in its current form, because that's what it is. An alternative way of having a bookmark menu in the browser bar.
Anothe clue is "Group", which is in singular. That's what you get. A single group, not several. I guess you didn't see that coming?

The browser tabs in Firefox, they stay open until you close them, further indicating this one is not about tabs.

As such, i e as "Browser Bar Bookmark List", it does a pretty good job in that it works as expected for what the name describes. I'd give it 5 for not messing anything up in that role. I'm not sure why anyone would need this, but hey it works! But with the initially mentioned facts I take two stars off.

It's possible if developed further, this could become what the name suggests. Some way of grouping tabs.