until today I used containers (out of the box without the extension) about:preferences#containers .
I configured two environments: Work and Private.
I tested the assignment feature and assigned one page to Private.
Then I had a look on the settings ( about:addons → Firefox Multi-Account Containers )
but I'm not able to find the list of either websites, domains or wildcards.
Then I opened about:config and searched for the domain of the website I assigned to the container Private
but unfortunately without results.
Thank you in advance.
The data on which site is assigned to which container is stored in the extension storage for the Multi-Account Containers add-on. You can extract that in bulk using the add-on debugging console, but I don't think it's (safely) editable there. The more arduous way is to use the interface to delete a site assigned to one container, and then add it to a different one.
Using the interface:
(1) Click the Multi-Account Containers toolbar button (not the one in the address bar, you may need to use the Extensions button drop-down). At the bottom of the drop-down panel, click "Manage Containers".
Firefox should bring up a panel with a list of your containers.
(2) Click a container name to view its details and then click "Manage Site List..." to view the assigned sites.
When you hover a site, a trash can will appear to remove it from the list.
I've looked for the ability to do this as well. As far as I know there isn't a way to edit the list of websites, which is unfortunate. It causes issues when using websites that use SSO, and one of the redirect URLs isn't in the same container as the destination URL. I'm forced to try to hit ESC mid SSO to stop my browser on the URL that I need to add to the container.