This is without a doubt the best Firefox extension out there.
This shouldn't be just an extension, it should be a core feature of Firefox.
Having this integrated into Firefox by default will bring probably the best differentiator Firefox can have on the market.

The only request I have is to make the container list hierarchical (closer to what bookmarks look like). Because at this point I have one container per site and it's hard to manage them.

And this could have so much potential to grow. Either do a client side suggestion engine where it creates containers for the sites you usually visit and suggest connections for sites that are part of the same user group but are on different domains (like maybe you want to connect Gmail with YouTube and with drive under one container).

Please don't ever give up on this feature. I hope you integrate it into core Firefox, but if not, at least work on improving this standalone addon. I think it's the last thing keeping me on Firefox. If this goes away, I don't know what browser will help me navigate the stormy oceans of the Internet.

Thanks for all your work!