Summary
The ::-moz-progress-bar pseudo-element applies to the area of an HTML <progress> element that represents the amount of progress that has happened so far. This lets you, for example, change the color of progress bars.
You have pretty broad capabilities to style the bar. This can be especially useful with indeterminate bars (via the :indeterminate pseudo-class), where you may wish to substantially change the default appearance; you can change the width and positioning of the bar, for example. (By which we probably mean the whole element's dimensions and position, not just the painted part of the element, as featured in the screen shot below.)
Examples
This example styles the progress meter portion of the progress bar with ID redbar with the color red.
#redbar::-moz-progress-bar {
background-color: red;
}
A bar using this style might look something like this:
