If I had to choose only one addon with my Firefox, it would be Tree Style Tab.

love it!

Excellent for me. Very customizable. Only slows down the pages even with multiple tabs. Without it the speed is OK.

SUPER.Ein echtes must have.Vor allen Dingen auf dem Laptop unverzichtbar.

I've been using this add-on for a few years and it helps me navigate the many tabs I tend to have open in my windows. I've never had a problem with this add-on.

Nach uBlock Origin die zweite Must-Have Erweiterung! Funktioniert top und hat unglaublich viele, sehr nützliche Funktionen!

Nicely takes the tabs to be stacked vertically on the left. With additional custom styles applied to the browser you can also hide the tabs in the title, so your browser has just the address bar on the top (and window buttons if on Windows), unfortunately this isn't supported in the extension itself out of the box but has to be done separately.

This is a very good addon, but there is a missing thing which is the following:
I do not want the appearance of the hierarchy between the tabs to be in the left side neither the right side

I want the hierarchy to appear in the top side, I mean in the original tab bar of Firefox, I mean like what was in the old versions, before converting the addon to the type of web extension
I mean
like what Piro have written in his website about this addon that this addon Shows the tab bar as a tree of tabs.

but I do not find this option in the preferences like before

I hope Piro (piro_or) update the addon with this feature

Amazing extension. It's so much better than any other mechanism for managing a lot of tabs. Essential if you do any sort of work that involves a lot of web searches and reading documentation, and excellent for personal use as well. I'm never going back to regular tabs. An important note is that the extension itself does not hide your regular tabs but it's easy to tweak Firefox to do so.