This is an excellent extension. I don't know why it has been hidden from search within the Mozilla Add-ons site. The only way to find it is to use a search engine.

This is one of the BEST and USEFULL Firefox add-on! I LOVE it :-) Thanks to all Devs from Mozilla Teams 👍

Fantastic. I love being able to segment at this level within a single browser. There are a few things that I wish to make the experience a little smoother such as opening a bookmarks folder in a container tab. However, that is small in comparison to the benefit factor.