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 introduces two new features to containers in Firefox.
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.
You may load it in a different container if you have set up a rule that allows the site to load in a different container. Other options on the page include disabling the entire rule set, or to enable the current container in the rule set.
Embedded content is blocked automatically.
Configuration is straightforward but unfortunately a bit clunky. Open the Firefox Add-ons Manager and select the Block sites outside container extension.
Select new rule to create the first rule. Rules use a combination of domain names and containers they are allowed to run in. Add a domain per line and select the containers you want the domain(s) to run in afterward.
A red border means that the sites are not allowed to run in that container; this is the default setting. You need to specify at least one container for the extension to work, unless you want to prevent a site from running in any container or outside of it.
Repeat the process to add new rule sets to the extension. You will notice that each is appended to the bottom of the page; manageability is not great if you set up a lot of rules but you may use the on-page find option of the browser to jump to specific rules quickly.
Another issue that you may run into in the current version is that the container listing gets truncated if the container count reaches certain thresholds.
Rule sets can be deleted or set to inactive at any time in the rules.
Block sites outside container is a useful extension for Firefox users who use Containers in the browser or did not use them because of missing functionality that the extension adds.
Setup is a bit complicated because of how rules are displayed and managed, but it is likely that the developer will address these issues in future versions.
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