Private mode will clear all data/history when you close the browser. So not sure what your asking here.

https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history

I never close the browser !

Hello

https://connect.mozilla.org/t5/ideas/you-need-to-be-able-to-tell-that-a-window-is-private/idc-p/56940/highlight/true#M42720 if you wish to test the preference browser.privatebrowsing.resetPBM.enabled Configuration Editor for Firefox https://support.mozilla.org/en-US/kb/about-config-editor-firefox search a preference name browser.privatebrowsing.resetPBM.enabled To change a boolean (true-false) preference's value, double-click on the preference to set its value to true

Don't have a clue what you're saying ! Can't you use layman's terms ?

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 default value false. Try true value.