JS判断IE6,ie7,ie8,ff

var isBrowser = function(vision) {
            if (typeof vision == 'undefiend')
                return false;
            vision = vision.toUpperCase();
            var isIE = !!window.ActiveXObject;
            var isIE6 = isIE && !window.XMLHttpRequest;
            var isIE8 = isIE && !!document.documentMode;
            var isIE7 = isIE && !isIE6 && !isIE8;
            if (isIE) {
                if (isIE6) {
                    return vision === 'IE6';
                } else if (isIE8) {
                    return vision === 'IE8';
                } else if (isIE7) {
                    return vision === 'IE7';
                }
            }
            else {
                return vision === 'FF';
            }
        }