::-moz-range-track CSS pseudo-elementNon-standard: This feature is not standardized. We do not recommend using non-standard features in production, as they have limited browser support, and may change or be removed. However, they can be a suitable alternative in specific cases where no standard option exists.
The ::-moz-range-track CSS pseudo-element is a Mozilla extension that represents the track (i.e., groove) in which the indicator slides in an <input> of type="range".
Note: Using ::-moz-range-track with anything but an <input type="range"> doesn't match anything and has no effect.
::-moz-range-track {
/* ... */
}<input type="range" min="0" max="100" step="5" value="50" />input[type="range"]::-moz-range-track {
background-color: green;
}A range slider using this style might look something like this:

Not part of any standard.
The pseudo-elements used by Gecko to style other parts of a range input:
::-moz-range-thumb represents the indicator that slides in the groove.::-moz-range-progress represents the lower portion of the track.Similar pseudo-elements used by other browsers:
::-webkit-slider-runnable-track, pseudo-element supported by WebKit and Blink (Safari, Chrome, and Opera)CSS-Tricks: Styling Cross-Browser Compatible Range Inputs with CSS