Ben Maurer & Nate Schloss:
So even if you've set proper expires headers for resources, hitting that reload button (which people must do a ton at Facebook) still requires server round trips to revalidate assets (sometimes).
They worked with Chrome:
After fixing this, Chrome went from having 63% of its requests being conditional to 24% of them being conditional.
Firefox implemented a proposal from one of our engineers to add a new cache-control header for some resources in order to tell the browser that this resource should never be revalidated.
So if you're using URLs for assets that never change (if they change, they'll be at a new URL) in Chrome you'll benefit automatically, and in Firefox you should use their new header.