When applying a dark theme to your gtk-system, Fx will use it to render form elements. This gets ugly in webpages and too often a page specifies a dark color for input elelments w/o specifying a light color for the background which makes the text invisible. This fixes those issues by always inserting default colors for form elements.

bright_forms.user.js

// ==UserScript==
// @name        bright forms
// @namespace   einar
// @description Make all form elements default to light style to combat gtk theme
// @include     *
// @version     1
// @grant       none
// ==/UserScript==
function addDefaultCssRules(rule) {
  var styleElement = document.createElement('style');
  styleElement.type = 'text/css';
  document.head.insertBefore(styleElement, document.head.firstChild);
  styleElement.sheet.insertRule(rule, 0);
}
addDefaultCssRules('input, button, textarea, select { background-color:#fff;border-radius:5px;color:#111 }');