What I’m trying to point out here, is that prefs declared in user.js (whether they are put there using scripting or otherwise) cannot be persistently modified at runtime from within Firefox. That may or may not be a huge problem, but something to be aware of.
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…