If this is about the zoom level for specific websites including builtin about: pages then this is about using permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History). When you use PB mode then all data is stored in memory and purged when you end the PB mode session (i.e. close all PB mode windows). This also happens if you would clear the Site Preferences, either via "Clear history when Firefox closes" or otherwise.

I don't know where the default Zoom level as set in "Options/Preferences -> General" is stored, possibly this is stored in local storage.