puts a Nexus S frame around your Firefox 4 content for mobile development

mobile-frame.css

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document domain("localhost") {
    html {
        margin-top: 129px !important;
        padding-bottom: 260px !important;
    }
    body {
        width: 320px;
        position: relative;
        left: -moz-calc(50% - 160px);
    }
    body:before {
        content: "";
        background: url(http://people.mozilla.org/~mclaypotch/nexus_s_frame.png);
        display: block;
        width: 408px;
        height: 785px;
        border: 500px solid black;
        position: fixed;
        top: -500px;
        left: -moz-calc(50% - 703px);
        z-index: 9999;
        pointer-events: none;
    }
    html:before, html:after {
        content: "";
        display: block;
        position: fixed;
        height: 100%;
        width: -moz-calc(50% - 160px);
        background: black;
        top: 0;
        left: 0;
    }
    html:after {
        right: 0;
        left: auto;
    }
}

drop this in Stylish or userContent.css and steup the @-moz-document domain() accordingly.