Hmmm .. I know that "browser.cache.disk.parent_directory" will only create a new location to store all the visited pages...
Now I shall ask you do something.
Create a image and upload it to photobucket. Again you edit the image and upload the same. Now try to open the image, it shows only the old (and the unedited image) only. I tried the same and I had to reload for 3-4 times untill I could see the edited image.
And now Press that Back/forward buttons, it's open without loading agian. Sometimes FX loads the pages and many of us have that issue with FX ; unlike Opera which has no issue.
And that 5% increase in the speeds is just my estimate
Just try to load some pages with heavy graphics concent and images like NDTV and try to load the same page again. It's definitely faster (atleast for me).