Hello
About, Private Browsing - Use Firefox without saving history.
https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history
Important: When Firefox is set to Always use private browsing mode or to Never remember history, you won't see a purple mask at the top of each window, even though you are in Private Browsing mode.
https://support.mozilla.org/en-US/questions/1535029#answer-1763552
To see, a trick to find out if you're use private window.
Type, about:cache, in the address bar.
Click on List Cache Entries, you'll find information privateBrowsingId.
Other indicator, to see, if you use private window.
1 - Go to Configuration Editor for Firefox https://support.mozilla.org/en-US/kb/about-config-editor-firefox
2 - Enter a search term browser.privatebrowsing.resetPBM.enabled, value false (by defaut), you can try true.
