When you go to your extension list (about:addons), do you see the Firefox Multi-Account Containers extension as enabled?

If you are currently in Private Browsing, note that this extension is not allowed to run in Private Browsing.

Thank you for the help! Is this a new policy? I believe in the past you were able to access it when in private browsing mode. When did this change?

It's specific for the Firefox Multi-Account Containers extension. I don't think you were ever able to access it in private browsing.

https://support.mozilla.org/en-US/questions/1292668#answer-1327615 https://support.mozilla.org/en-US/questions/1330109 https://github.com/mozilla/multi-account-containers/issues/1864