I use it all the time. i can see in a glance which tab is parent of another. and I can jump from the first to the last in a click, evne if i have more than 100 tabs opened.

Didn't like it, removed.

After using it, now I need this in every browser I use.
Side note: If someone wants to hide upper tabs, they can do so using custom css or install addons.mozilla.org/en-US/firefox/addon/toggle-native-tab-bar/

Overall looks great and works well, but when it's active I can't right-click on tabs to bookmark them. When I try to do this, nothing happens. I tried creating a new profile and the problem is still there. Only when I disable the extension am I able to add bookmarks in this way.