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Block sites outside container is a new extension for the Firefox web browser that extend the container functionality of the browser.

Containers, first introduced as a Test Pilot experiment for Firefox and later on released as the Firefox Multi-Account Containers extension, allow users to separate sites and local date from each other. Useful applications include using Containers to sign-in to multiple accounts, e.g. Gmail, Twitter, or Facebook, at once, or to limit tracking.

Another thing that makes the container functionality great in Firefox is that extension developers may improve it. We reviewed several extensions designed to improve Container functionality in the past:Temporary Containers, Facebook Containers, Containers with Transitions, or Taborama are just a few of the extensions that do so.

Note that you don't need the Multi-Account Containers extension to use Block sites outside container. You cannot create or manage containers using it though.

Block sites outside container

Block sites outside container introduces two new features to containers in Firefox.

1. Block any site from loading outside their designated container(s).
   
   
2. Allow sites to run in multiple containers.
   
   

The second use case is explained easily. You can force sites to open in specific containers using the Firefox Multi-Account Container extension. What you cannot do using it is specify that you want sites to run in multiple containers only.

Block sites outside container changes that as you may set up rules to allow sites to run in multiple containers exclusively.

The blocking option prevents sites from loading outside their designated container(s). Means: any attempt to load embedded content, say a YouTube video, Twitter or Facebook buttons, or other embedded content, fails.

The extension displays an error page when you attempt to load a site that has been blocked in a particular container.