Doesnt seem to have changed e.g. on this forum its instant from cache.
But some thoughts.
in v32 of firefox they new cache system is on by default (I am using esrv31). Its possible this new cache system acts differently on the back button.
many pages have dynamic content that isn't cached, such as adverts, widgets etc. These will refresh when the back button is used. for that reason I tend to block things like twitter widgets.
There is an option in about:config where can choose to effectively always load the cached version of everything unless you force reload, flush cache or relaunch the browser. But although it sounds great you will also be loading outdated pages when browsing things like forums, so will find yourself e.g. force reloading to get new posts up on a forum. The option basically ignores the ttl and expires headers of web content.