This will fail with Gecko not being followed by a year, but a version number like in… Firefox OS and Firefox Android. Found in http://static0.viadeo-static.com/resource/308570/tetra-viadeo-VNS.js

xmlhttprequest.js

setRequestHeaders: function() {
    var a = {
        'X-Requested-With': 'XMLHttpRequest',
        'X-Prototype-Version': Prototype.Version,
        Accept: 'text/javascript, text/html, application/xml, text/xml, */*'
    };
    if ('post' ==
        this.method && ('undefined' === typeof a['Content-Type'] && (a['Content-Type'] =
                this.options.contentType + (this.options.encoding ? '; charset=' + this.options.encoding : '')
            ),
            this.transport.overrideMimeType && 2005 > (navigator.userAgent.match(/Gecko\/(\d{4})/) || [0, 2005])[1])
    ) a.Connection = 'close';
    if ('object' == typeof this.options.requestHeaders) {
        var b = this.options.requestHeaders;
        if (Object.isFunction(b.push))
            for (var c = 0, e = b.length; c < e; c += 2) a[b[c]] = b[c + 1];
        else $H(b).each(function(b) {
            a[b.key] = b.value
        })
    }
    for (var f in a) this.transport.setRequestHeader(f, a[f])
},