This extension does not work. Try the following on firefox. Log into an Amazon account in a container in one tab, then open another container tab and log in to another separate Amazon account. Go back to the first tab and refresh the tab. The result is that the first tab now shows the account info of the second tab Amazon account. So in effect these multi account containers tabs do not isolate anything at all.
That's definitely a bug if the tabs are separate containers. Are you comfortable filing a new bug (or up-voting an existing bug) at the GitHub project: https://github.com/mozilla/multi-account-containers
