https://github.com/arkenfox/user.js
Firefox privacy, security and anti-tracking: a comprehensive user.js template for configuration and hardening - arkenfox/user.js
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?
Sure. For simplified example have only the following in your user.js file:
user_pref("browser.tabs.warnOnClose",true);
Confirm before closing multiple tabs is checkedbrowser.tabs.warnOnClose is now falsetrueThe reason is also very simple. Firefox will never write anything to user.js - thus any changes you do at runtime will only be stored to prefs.js. However, user.js always overrides prefs.js at startup.
Understood, thanks. So on a clean install, I'm assuming user.js is either empty or missing, correct?
Yes. Firefox doesn't create user.js file itself - if you want one then you need to create it yourself either manually or with some tool. Also, I've seen some "security" software create user.js file without notifying the user about it...