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