Firefox doesn't save the History privacy setting
Settings/Privacy & Security/History
When I set "Firefox will use Custom Settings for History" to "Firefox will never remember history" Firefox does not save that setting, and reverts to Custom Settings after restart. The 'Always use Private Mode' is checked. When I check Cookies & Site Data it shows I'm using cookie space even though every window has been opened in Private mode.
I tried this on a new Firefox install after deleting the ~.mozilla, ~Library/Mozilla, and ~Library/Application Support/Mozilla directories. No extensions or themes were installed.
Firefox 112.0.2 (64bit) MacOS Ventura 13.3.1
Opaite Mbohovái (2)
When your in Private mode cookies and settings will be active until you close the browser then all settings a cleared along with any cache and data during that activity.
Choosing "Firefox will never remember history" will enable Private Browsing mode and "Always use Private Browsing mode" gets a chackmark. However Firefox will revert to showing "Use custom settings for history" if there are changes made from the default history settings to make you aware. "Use custom settings for history" as such doesn't make changes, but merely allows to inspect the current settings.