Containers is a new feature of the Firefox web browser that is currently being tested and in active development.
The feature allows you to launch websites and services in containers to separate them from one another in various ways.
You may use the feature to separate work from entertainment sites for instance. Some browsing data is restricted to the container it is generated in. This is the case for cookies for instance, so that you may open the same site in different containers to sign in to different accounts at the same time without having to use different profiles or other means for that.
Since cookies are separated, it reduces the tracking impact of cookies as sites cannot access cookies in different containers anymore.
Always open in Container in Firefox
When Mozilla launched Containers in Firefox Nightly in mid 2016, it revealed plans to improve the functionality of containers over time.
One of the features on that list was an option to launch sites always in specific containers. The feature made a lot of sense, considering that you may want to launch news sites always in news containers, your bank's website in the banking container, or entertainment sites in the entertainment container to separate them always from anything that is not run in the container.
The feature has launched as an update for the Test Pilot Containers add-on for Firefox. If you have not already, you may download Test Pilot and the Containers experiment to get the full functionality.
The current work flow requires that you open a site in a container, and assign it to the container afterwards.
You can launch sites by right-clicking on links for instance to select "Open Link in new Container Tab" to launch it in one of the available containers.
Once the site has been launched in the container, right-click anywhere in the content area, and select the "Always open in this container" option from the context menu.
A prompt is displayed the next time you open the site, be it with a click on a link that points to it, or by typing the address manually in Firefox's address bar.
To open the site in a container, click on the take me there button. You may check the "remember my decision for this site" option to always open it in the container without the prompt being displayed again to you.
Note: Site means entire domain in this regard. Any page on the domain will be launched in the container if you select that option.
You may remove the assignment at any time by repeating the process. In short: open the site in the container, right-click on the content area, and remove the checkbox from "always open in this container".
Firefox displays a desktop notification each time you add or remove sites to or from containers.
The option to link sites to containers so that they are opened in the linked containers is a welcome addition to the feature. The main reason for that is that it ensures that a site will be opened in a selected container, and not anywhere else.
You get added benefits from this. Phishing sites won't be opened in the same container by default for instance, as they use a different domain than the copied site. You ensure as well that the site's cookies stay in that one container.