During a reload all cache resources will be revalidated. In many cases the response is 304 Not Modified and the cached resource is used.
Some years ago we decided to specify and implement Cache-Control: immutable [0, 1]. The resources with the directive will never be re-validated. This eliminates unnecessary requests for resources that will never change.
On the other hand, Chrome implemented a relaxed re-validation policy for soft reload, i.e. subresources will not be revalidated (if they are still valid) regardless of the "immutable" cache-control.
The difference in behavior between Firefox and Chrome has been tested here . A soft reload is widely used as the data in this comment show .
We have decided to relax the soft reload behavior in Firefox. The behavior will be controllable by a pref with the relax behavior being the default one.