If you recently installed the latest Mozilla Firefox update and are missing the icons on your intranet site, this is due to security changes implemented my Mozilla.
In the version 87, Firefox changed the default referrer policy to a more privacy-preserving setting.
If this is causing an issue with icons or if you are getting any other 401 errors on the site, there is a way to change this in the hidden Firefox settings.
Steps to Resolve
Type "about:config" in the browser URL field and click on "Accept the risk and continue" button:
Type in "network.http.referer.defaultPolicy" in the search field and edit the first option that shows:
Change the value for 2 (Default) to 3 and save the change. Afterwards, restart the Firefox browser and icons should now load on the site.