@-moz-document url-prefix() {
  .selector {
     width:200px;
  }
}

@media screen and (-webkit-min-device-pixel-ratio:0) {
 .selector {
     width:300px;
  }
}

if (!!window.chrome == true) {
  //condition for chrome
 }
 else if (typeof InstallTrigger !== 'undefined') {
 //condition for firefox
 }
 else if (/*@cc_on!@*/false == true) {
       //condition for safari
    }
    else if (!!window.opera || navigator.userAgent.indexOf('Opera') >= 0) {
        //condition for opera
    }
    else if (Object.prototype.toString.call(window.HTMLElement).indexOf('Constructor') > 0) {
        //condition fo IE
    }

<script type="text/javascript">
    var browser=navigator.appName;
    if browser == "Microsoft  Internet Explorer"
    {
        document.write("<link type="text/css" rel="stylesheet" href="IE.css">");
    }
    else if browser == "Firefox"
    {
        document.write("<link type="text/css" rel="stylesheet" href="Firefox.css">");
    }
    else
    {
        document.write("<link type="text/css" rel="stylesheet" href="generic.css">");
    }
</script>

<!--[if IE]><link href="/ie.css" rel="stylesheet" type="text/css" /><![endif]-->