That sounds like a nightmare. Maybe one of the many related Microsoft sites is in a container, isolated from the others? It's unfortunately quite a bit of work to track this down because the site lists are per container. Unless someone knows a faster way:

• Click the toolbar button to drop the container list
• Click the > next to the first container
• Click the button at the bottom of the next panel labeled Manage This Container
• Click the Manage Site List... link
• Review/remove sites that do not belong in this container
• Repeat with your other containers

(Overly wide screenshot attached)

Thanks! I never saw that area in the options. In the meantime I discovered that I could turn the container extension off and get into my Outlook mail, then I was able to turn the container extension back on and Outlook website continued working and containers were back. What crazy nonsense. Not to mention annoying. It only started happening a couple of weeks ago; all my searches had complaints that were much older. Only one described exactly what I was experiencing and there was no way I was going to try to reinstall Firefox, reinstall my extensions, bookmarks, etc., which may or may not work anyway.

I think there is some kind of complicated cookie/token handoff among Microsoft sites to get logged in, but after that, whatever the problem site is/was isn't a factor any more. Or something... it goes by a bit too quickly to follow.