Been working on different apps for FirefoxOS, which I'm loving by the way.
For my latest app I've been using jQuery Mobile which, to my surprise, doesn't come with a progress bar control.
Luckily for you, here's the code to generate yours with as few lines of code as possible.
This code is being used for the landing page of this app while it loads some stuff on the background.
You can put the slider's button back by removing the .ui-slider-handle from the CSS