Banks often use a large number of different URLs (subdomains) which change frequently. It can be difficult to make sure all of their servers end up in the same container when needed, and that breaks the cross-site connections needed to use their pages.

As the first step, you may need to remove all the URLs from the banking container. Click the Multi-Account Containers toolbar button and click Manage Containers at the bottom of the drop-down panel. Then click Banking. Then click Manage Site List... Hover each URL you want to remove and a trash can should appear on the right side allowing you to unassign it from the container.

Does that help?

Thanks for the suggestion. As it happens, I have already deleted all the sites in the Banking container and then put in my credit union in the container. Then I noticed that Firefox had populated the container with all the credit card, savings banks, and other financial institutions I do business with. All work fine. Only the Citibank credit card will not log.

I think the problem started when I misunderstood an on screen message when going to citibank and opened it outside the banking container. I have no idea how to correct that mistake.

I just tried again to open Citibank without first specifying the Banking container. Firefox reminded me that I had asked to put that site into the Banking container. I had to choose, so I did. After that it worked. Now I can open the site in the Banking container without a problem. Perhaps my mistaken attempt to open it outside the banking container created a contradiction.

It's interesting that Firefox detected financial sites. Or maybe the add-on has a list.