Dark GTK themes mess up some poorly styled webpages; fix that with a Stylish usertheme.

dark_gtk_fix.css

input:not(.urlbar-input):not(.textbox-input) {
  -moz-appearance: none !important;
  background-color: white;
  color: black;
}

#downloads-indicator-counter {
  color: white;
}

select {
  -moz-appearance: none !important;
  background-color: white;
  color: black;
}

textarea {
  -moz-appearance: none !important;
  background-color: white;
  color: black;
}