This is a companion add-on for
Firefox Multi-Account Containers which goes a step further and allows you to block all content for specific sites from loading except in their designated containers. That way, blocked sites that are embedded in sites loaded in other containers (e.g. Facebook "Like" buttons, Twitter buttons, etc.) cannot see what you are doing outside their containers.
Another issue this solves is that althought Multi-Account Containers allows you to specify that a site should always open in a specific container, it does not work if you want to use that site in multiple containers, for example, to manage multiple accounts. With this extension, when a site is blocked, you can choose from multiple containers to load the site in.
