Another option is Sidebery, seemed to be the only maintained one when I last looked. It also supports 'panels' - 'tabs' to the sidebar that switch between sets of tabs. You can assign a panel to a FF container.

This looks like a good replacement of the old tab group Firefox used to have. The fact we can assign them a container is really promising, I'll give it a go!

I found this[0] minimal style (not by me) particularly useful, with auto-folding and a thin visual footprint.

[0] https://github.com/ongots/firefox-sidebery-minimal-style (screenshots further down the page)

For me it replaced both my tab groups extension and vertical tabs extension. It's a godsend, although a little bit buggy every now and then.

This. The Vivaldi implementation sucks because: not a tree.