Firefox supports loading about:config settings using user.js file that does not support even the basic thing Javascript is supposed to support
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
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.
It just looks like a JavaScript file. Once upon a time in Netscape 3 and maybe 4 it actually was, but now it's just a file with a .js extension and a very restricted syntax that's parsed by a separate (non-JS) parser and not executed in any way.