There are many websites that shallowly specify font-family: system-ui in CSS. For systems that do not allow a choose system font, this may degrade the user's experience. In about:config it can be disabled by layout.css.system-ui.enabled. Could the default value be set to false or changed in about:preferences?
Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.