<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>

<window xmlns:html="http://www.w3.org/1999/xhtml"
        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
        >

<html:style>
    #prog {
        -moz-appearance: none;
        background-color: blue;
    }
    #prog .progress-bar {
        -moz-appearance: none;
        border-radius: 3px 3px;
        background-color: red;
    }
</html:style>

<progressmeter id="prog" value="50" max="200"/>

</window>