How to interpret grey items in Firebug Net panel?

Question

I\'m trying to debug some tricky behaviour in my web application: It appears as though an XHR GET request is being sent by the client, but the server never actually receives

Answer 1

"Firebug color codes requests that are served from the cache in a lighter gray..."

So the reason the server doesn't see the request is that the client never actually sends it, it simply receives the response from its cache. If you're using jQuery, there is a cache property that you can use on ajax() to prevent AJAX requests from being cached. If you're not using jQuery you can append a dummy parameter to the end of the request URL that has a value of the current time in milliseconds (this is in fact what jQuery does I believe).

url + "?v=" + (new Date()).getMilliseconds()

This should ensure the URL is always unique and prevent the browser from using caching.