Firefox shipped with support for Cache-Control: Immutable:
The benefits of immutable mean that when a page is refreshed, which is an extremely common social media scenario, elements that were previously marked immutable with an HTTP response header do not have to be revalidated with the server.
No more 304‘s for those resources, because the browser won’t even re-request them 🙂
Using Immutable Caching To Speed Up The Web →Cache-Control: immutable introductory post →