Why use .js extension if it does not even support what Javascript is supposed to support. Here is my user.js file that does not work in Firefox: https://pastebin.com/QZXL0YJp
However the following works: https://pastebin.com/xEjh7g9U
I wonder why not implement a full Javascript engine to read in those configuration files, it kind of sucks to keep on typing user_pref( in every line when one could put all the configs in a list and loop over it and use user_pref() to apply them.
Code execution increases security risk which requires more work to make sure they're doing it right. Better to ship something than nothing