I have seen this on a lots of websites e-g: Even here:
http://www.w3.org/2005/10/howto-favicon
When you load all the resources for this page under Net Panel in firebug and check the images tab, it shows all the images on the page but the favicon doesnt show up. Why?
Should I be looking for it somewhere else in firebug.
The first answer to this thread, by Jan Odvarko (who is a contributor to Firebug), seems to explain why the favicon is not displayed in Firebug's net tab (quoting):
The problem is that the network request for favicon isn't associated with the page where the icon is displayed.
So, Firebug (i.e. Net panel) don't know that it belongs to the page and skips the request
来源：https://stackoverflow.com/questions/10667348/why-doesnt-the-favicon-icon-show-in-firebug-net-panel