ayuda

[Raulmmmm]

Es un poco largo, pero funciona:

Código HTML:

<script type="text/javascript">
//detección del navegador por http://www.quirksmode.org/js/detect.html
var BrowserDetect = {
       init: function () {
          this.browser = this.searchString(this.dataBrowser) || "An unknown browser";
          this.version = this.searchVersion(navigator.userAgent)
             || this.searchVersion(navigator.appVersion)
             || "an unknown version";
          this.OS = this.searchString(this.dataOS) || "an unknown OS";
       },
    searchString: function (data) {
       for (var i=0;i<data.length;i++) {
          var dataString = data[i].string;
          var dataProp = data[i].prop;
          this.versionSearchString = data[i].versionSearch || data[i].identity;
          if (dataString) {
             if (dataString.indexOf(data[i].subString) != -1)
                return data[i].identity;
          }
          else if (dataProp)
          return data[i].identity;
       }
    },
    searchVersion: function (dataString) {
       var index = dataString.indexOf(this.versionSearchString);
       if (index == -1) return;
       return parseFloat(dataString.substring(index+this.versionSearchString.length+1));
    },
    dataBrowser: [
       { string: navigator.userAgent,
          subString: "OmniWeb",
          versionSearch: "OmniWeb/",
          identity: "OmniWeb"
       },
    {
       string: navigator.vendor,
       subString: "Apple",
       identity: "Safari"
    },
    {
       prop: window.opera,
       identity: "Opera"
    },
    {
       string: navigator.vendor,
       subString: "iCab",
       identity: "iCab"
    },
    {
       string: navigator.vendor,
       subString: "KDE",
       identity: "Konqueror"
    },
    {
       string: navigator.userAgent,
       subString: "Firefox",
       identity: "Firefox"
    },
    {
       string: navigator.vendor,
       subString: "Camino",
       identity: "Camino"
    },
    { // for newer Netscapes (6+)
       string: navigator.userAgent,
       subString: "Netscape",
       identity: "Netscape"
    },
    {
       string: navigator.userAgent,
       subString: "MSIE",
       identity: "Explorer",
       versionSearch: "MSIE"
    },
    {
       string: navigator.userAgent,
       subString: "Gecko",
       identity: "Mozilla",
       versionSearch: "rv"
    },
    { // for older Netscapes (4-)
       string: navigator.userAgent,
       subString: "Mozilla",
       identity: "Netscape",
       versionSearch: "Mozilla"
    }
    ],
    dataOS : [
    {
       string: navigator.platform,
       subString: "Win",
       identity: "Windows"
    },
    {
       string: navigator.platform,
       subString: "Mac",
       identity: "Mac"
    },
    {
       string: navigator.platform,
       subString: "Linux",
       identity: "Linux"
    }
]
};
BrowserDetect.init();

if (BrowserDetect.browser != "Firefox") {
    document.write('<a href="http://www.mozilla-europe.org/es/firefox/"><img src="http://www.difundefirefox.com/files/banners/180_60-ff1.png" alt="Descarga Firefox" /></a>');
} 
</script>