I think that the Facebook Container extension currently lacks an easy way to remove associated domains that you have saved. However, if this is the only domain that you have saved to the extension, there is a more technical way that we can remove it from the list.

First, go to the about:debugging page in Firefox. On that page, select This Firefox from the left-side of the screen. There should be a list of the extensions that you have installed on Firefox.

On that list, there should be the Facebook Container extension and you should be able to click the Inspect button for it. That will open the developer tools in a new tab.

If it's not already selected, go to Console at the top of the page. This is the section where certain error messages and technical data is displayed, but we can also enter lines of code in this window to run on the extension.

In this case, you remove all domains that been associated with the extension (excluding of course the default Facebook-owned ones), you can copy and paste the following line of code:

browser.storage.local.set({domainsAddedToFacebookContainer: []});

NOTE: It's a pretty long line of code, so it may not fully display on this website. It might appear in a scrollable section on this page, so make sure that you copy all of it. Triple clicking on the line of text above is probably the easiest way to select the entire line.

Once you have pasted that into the console window, you can press the Enter key on your keyboard. When you do that, you may get some text that refers to a Promise. That's perfect.

Then, if you close all Firefox windows and open Firefox again, that domain should be removed.

Hope this helps. Sorry it's not the most user-friendly solution, but I don't believe that there is an easier way to do this at the moment.

Thanks very much, feel more secure now.

HUGE clarification required on my part. While the solution that I provided above is not technically wrong, the people that developed the Facebook Container extension have actually provided a much easier way to do this, it's just slightly hidden.

To do this, you need to have the Facebook Container button on your toolbar. See Customize Firefox controls, buttons and toolbars for more information on how to add/remove items from your toolbar.

When you are on a website that you have allowed to use the Facebook container, you click the Facebook Container button on your toolbar and you should see a popup panel. That popup should have an option called Sites Allowed in Facebook Container.

If you click that, you can see the list of sites that you have allowed and can remove them from there. I'm providing this clarification just in case anyone else with a similar question stumbles across this thread.