The code snippet below inserts the pink div between the track and thumb of the range slider.

Why does it work in other browsers (-webkit-slider-thumb), but not Firefox (-moz-range-thumb)?

Element inspector says z-index has no effect because it’s not a positional element, but the position has been set to relative:

Note: Please see my first question (that was closed) for full motivation, context, and code: How to adjust z-index of pseudo-element ::-moz-range-thumb?

main {
  position: relative;
  width: 200px;
  height: 50px;
}

div {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 50%;
  right: 25%;
  background-color: pink;
}

input {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
}

input::-webkit-slider-thumb {
  position: relative;
  z-index: 1;
}

input::-moz-range-thumb {
  position: relative;
  z-index: 1;
}

<main>
  <input type="range" />
  <div></div>
</main>