FirefoxPrivacyConfig VS user.js

By "firefox privacy oriented config", did you mean this? Tenmag/FirefoxPrivacyConfig on GitHub

I don't know about Chrome based browsers but if you are using Firefox you can try user.js from arkenfox [link] or from pyllyukko [link] (I personally recommend the first one because they have a great wiki), you just put the script to your Firefox profile directory and it will spoof your screen resolution, link referrer, user agent, etc. (Remember to manually edit the file to suit your preferences, otherwise it may delete your history and cookies!). Also you can try User-Agent Switcher and Manager [Chrome] [Firefox] it is open source and lets you change your user agent to whatever you want. You can view your browser information on this website: https://deviceinfo.me, IMO this is a great website for identifying your browser's privacy issues.

I don't know how this one compares to Arkenfox, but from my limited understanding, it seems it's more/less of an out-of-the-box Arkenfox experience.

I recommend pyllyukko's hardened user.js https://github.com/pyllyukko/user.js/

I've been using the user.js script based on https://github.com/pyllyukko/user.js/tree/relaxed for a few years now, but today I noticed that it hasn't been updated for quite a while.

If you're that worried about privacy you should probably change your Firefox to one of these templates.

Already tried it. Pretty nice but I prefer hardening Firefox on my own (which just means copying and pasting this user.js accordingly and applying some extensions )