Ugh. Because they also say:

"Two other forms of advanced configuration allow even further customization: about:config preference modifications and userChrome.css or userContent.css custom style rules. However, Mozilla highly recommends that only the developers consider these customizations, as they could cause unexpected behavior or even break Firefox. Firefox is a work in progress and, to allow for continuous innovation, Mozilla cannot guarantee that future updates won’t impact these customizations."

https://support.mozilla.org/en-US/kb/firefox-advanced-custom...