It is not recommended to copy a full prefs.js to other profiles. The prefs.js file stores all preferences that do not have a default value or aren't present by default and this includes profile specific prefs and timers for stats. If you want to transfer prefs, best is to copy relevant lines from prefs.js to a user.js and use this file to initialize a profile. The user.js file is processed each time you start Firefox, so you should remove this file after it has transferred the prefs or you won't be able to modify those settings permanently.