Hard to love when the broken extension ecosystem prevents the use of vertical tabs for these scenarios.
Here's an in-development vertical tabs extension using their new extensions API.
https://addons.mozilla.org/en-US/firefox/addon/tab-center-redux/
Apparently the tabs still show up at the top because the API to disable that isn't available yet. But you get vertical tabs and it should get better over time. I've been using this and it isn't 100% perfect, but getting better and better.Sadly, the new extension model isn’t nearly ready for prime time yet.
They changed their add-on API and are in the process of deprecating the older one which allowed for radical UI changes. That's probably what he was talking about.