Looks like this does not have the hierarchy/tree organization of sidebery, just the verticality.

I started and manage a relatively popular Firefox modification that has vertical tabs (with dynamic indentation, ie unfurls on hover) for several years now. The latest version now (ab)uses the new vertical tab sidebar to make Sidebery work rather well (and _massively_ reduce the complexity of the older versions!)

I'll link it here, since it might be of interest to someone: https://github.com/drannex/FirefoxSidebar

Looks nice! I have a few questions after watching the video in the readme if you don't mind:

In sidebery my pinned tabs are always at the top, but it looked like the pinned tabs in yours are part of the scrolling area? One thing I like a about pinned tabs always being visible is being able to see when I have new email/slack/calendar notifications even if scrolled way down the tab list. Is that possible with yours?

I usually have 500-1000 tabs, have you tested for any potential usability bugs when having way more tabs than fit vertically on one screen?

I need to update the video... It still shows the original (now legacy) TST implementation, but the gist and general design is the same.

Yes, the pinned tabs float (are 'stuck') to the top of the container/sidebar when you scroll.

I have hundreds of tabs at a time, and it works perfectly well.