Firefox gets httpOnly cookies
httpOnly cookies allow you to hide your (session-)cookies from javascript. In the case of an XSS hole in your application, it will make a hackers life much harder to steal someones session.
Internet Explorer support httpOnly cookies for a long time, but since version 2.0.0.5 Firefox also supports this feature. Apparently Mozilla hasn't openly promoted this new feature yet, because its still possible to fetch the cookies with XMLHttpRequest. PHP has support for httpOnly cookies and sessions since 5.2.