I installed Firefox Multi-Account Containers and found it working OK, on most pages. On some however it prevents me from logging in, for example on https://login.blau.de (German mobile phone provider) I end up consistently in an error message that "for technical reasons this website is not available". I don't get this error when I login with a browser WITHOUT Firefox Multi-Account Containers enabled. Why do I run into an error when logging in on some sites, but not on all? How can I disable MAC for specific websites?