The way tree style tabs worked after they broke it was never very good. Floorp is what to use if you wanted side tabs on Firefox.
That said I still went back to Vivaldi after trying to use Floorp because of stupid little ux issues like pinned tabs not being protected from closing, and broken session saving.
The issue is that because they broke the UI customization that allowed for it all the extensions are just a kludge to add a panel to the side without actually getting rid of the top tabs.
The way tree style tabs worked after they broke it was never very good. Floorp is what to use if you wanted side tabs on Firefox.
That said I still went back to Vivaldi after trying to use Floorp because of stupid little ux issues like pinned tabs not being protected from closing, and broken session saving.
Sidebery is a very good implementation of the vertical tab panel
The issue is that because they broke the UI customization that allowed for it all the extensions are just a kludge to add a panel to the side without actually getting rid of the top tabs.