How can I hide a div with javascript if the browser is firefox only?
Just check a FF-specific JavaScript property. E.g.
var FF = (document.getBoxObjectFor != null || window.mozInnerScreenX != null);
if (FF) {
document.getElementById("divId").style.display = 'none';
}
This is called feature detection which is preferred above useragent detection. Even the jQuery $.browser API (of which you'd have used if ($.browser.mozilla)
for) recommends to avoid useragent detection.