I prefer Librewolf as it is easier and simpler to use
I don’t think that could work. Not unless we are talking about different things, or unless you run their updater script everytime before starting Firefox.
If you use user-overrides.js, it adds your custom preferences at the bottom of the user.js, as the prefs are read from top to bottom, if a new duplicate exist in your user-overrides.js but with a different value the new value would be used as it is at the bottom.
Yes, but that is not what I’m talking about. What I mean is that when Firefox is running and you go to change some setting in say, Settings page, then the new value for that preference is stored into prefs.js (at latest on Firefox shutdown, it might remain only in-memory for some time I’m not sure). Anyway, the new value persists only for that browser session, because on next startup whatever value was set by user.js will override it.
Have you independently confirmed this?
What is preventing user.js
from doing exactly what you’re describing right now on your system?