This is an excellent Add-on!
In future versions of Firefox it should come preinstalled and be easier to use.
Some things that might help:
- Add a "new container" button beside the "+" (new tab) button.
- Allow users to edit which websites are managed by each container directly from the add-on, without having to navigate there.

I cannot emphasize this enough: make this default and give it more visibility, otherwise only a bunch of us nerds will use it.