Remember, when software UI was customizable? Developers somehow managed to build amazing customization options while adhering to OS design and QA had it all tested without any fancy "automated testing tools" we have today.
Firefox needs to figure out how to expose UI customization to all users, not only those who are comfortable enabling user styles and hacking CSS. Most UI styles probably are using CSS variables, so how about starting to expose them in some easy to use interface, with simple textbox or spinner? Of course, they should plop a huge banner, that customizations are not supported, but in the end this should help users to quickly fix their personal annoyances. It is impossible to make single UI that will satisfy all users, but such customization tool surely will turn more grumpy users to happy users. Imagine, how disabled person must feel, when they need to go through 10 steps of browser customization just to adjust a single color.