Firefox (Quantum) with dark GTK themes

The Problem

Having dark GTK themes active results in Firefox (Quantum) rendering forms and other stuff unreadable.

The Solution

Replace the css for all relevant elements!

Create a file ~/.mozilla/firefox/blergbaz.default/chrome/userContent.css with the following content:

(You have to replace blergbaz.default with your profile name, obviously.

button,
input:not([type="checkbox"]):not([type="radio"]),
select,
textarea {
  -moz-appearance: none !important;
  background-color: white;
  color: black;
}