I've been working on adding support for <input type=range> to Mozilla. This work is progressing well and <input type=range> is now turned on in the latest Nightly builds to help gather feedback from content authors. If you're interested in <input type=range> I'd love it if you could try it out and report any problems/make enhancement requests. Be sure to mention "<input type=range>" in the Summary field of any reports that you file, and add ":jwatt" to the CC field. I'm currently on vacation but I'll work through any issues that are reported once I get back next week.
To allow content author's to style <input type=range> there are currently two pseudo-elements, ::-moz-range-track and ::-moz-range-thumb. Very rough WIP documentation is here.