The add-on is great! I can imagine that it's a bit difficult to understand if you are not that much into computers though. The only thing that I can not find is a way to manage which tabs belong to which containers. You can assign them, but you cannot manage them.

If that "feature" is added, or some one of Mozilla can explain to me how I can manage it, I'll update this review to a 5-star.

EDIT: I've found it, but why is it so hidden?
It is hidden 6 clicks deep within the extension's toolbar button. Why not put that also in the settings of Firefox add-ons menu?