Mozilla broke the observatory's detection of private browsing mode
Error ``CC['@mozilla.org/privatebrowsing;1'] is undefined_ [x-] in file ``file:///home/pde/.mozilla/firefox/xx18zmvu.httpse%20testing/extensions/https-everywhere@eff.org/components/ssl-observatory.js_, line 394, character 0.
Documentation of the problem is here:
https://developer.mozilla.org/EN/docs/Supporting_per-window_private_browsing