Currently, any time you press back or forward, the url bar displays the loading spinner and the page seems to load again from scratch. Going back or forward to a page that is already cached in a users session history should feel as instantaneous as possible. This should include 1. Instantly returning the URL bar to the previous page's state (favicon / lock / title) with no loading spinner in between 2. Instantly displaying the previous page's content
Seems to WORKSFORME as long as the bfache is enabled, for which see bug 1335148.