about:config controls most of Firefox settings, from network behavior, fingerprinting defense to the order of extension icons on the toolbar.
The last example is a browser interface change that has no impact on browser fingerprint: you can add or remove an extension icon or settings icon in the toolbar without impact.
Meanwhile another interface change like adding a bookmark toolbar will change your page content size (also called viewport).
In this case, it will change your browser fingerprint but it’s ok because the page content size is protected by the letterboxing.
As you can see, without deep knowledge it’s impossible to know what you’re doing in about:config, and it’s easy to shoot yourself in the foot, so the recommendation is not to change anything.