if (/Firefox[\/\s](\d+\.\d+)/.test(navigator.userAgent)) { //test for Firefox/x.x or Firefox x.x (ignoring remaining digits);
    var ffversion = new Number(RegExp.$1) // capture x.x portion and store as a number
    if (ffversion >= 3) {
        //document.write("You're using FF 3.x or above")
    } else if (ffversion >= 2) {
        document.write('<link rel="stylesheet" href="/styles/web/FF2.css" type="text/css" media="screen" id="screenStyles" />');
    } else if (ffversion >= 1) {
        //document.write("You're using FF 1.x")
    }  
}