Comment by blackbear_

Have you tried Tree Style Tab? https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...

It moves the tab view to the side, and each tab you open is grouped "under" the current tab. So you can also have nested groups up to any depth, and you can fold groups you aren't using. It's great to keep several rabbit holes nicely organized next to each other ;)