How are deleted-prefrences managed?

[KAGEYAM4]

131.0...133.0

In the above, under user.js we can see that whole preference is removed. So how does it gets reset to default? Or is it like this -->

• If a preference needs resetting, it is set to another value instead of deleting the whole line
• If it is not that major settings wise, it will just be removed from user.js whereas the prefrence is still saved in prefs.js by firefox.