[NoGravitas]

There's a Tree Style Tabs-like WebExtension. It has a few quirks, though. It runs in the sidebar. It doesn't hide the top tabs (though you can do this with Stylish). It has its own menu for tabs which does not include some options (like send tab to device).

There was a very good non-nested side-tabs extension in Firefox Test Pilot, but it has expired.

[vetinari]

Tab Center (the extension from Test Pilot that expired) is not an WebExtension. It is a classic extension that will stop working in FF56.

Yes, I'm using it ;)