Memory cache problem

Hello,

I´m making a video manual, but i don´t want to be cached in the client
machine.

I make a script like this:

<?php
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
header("Cache-Control: no-store, no-cache, must-revalidate");
header("Cache-Control: post-check=0, pre-check=0", false);
header("Pragma: no-cache");

readfile($video_name);
?>

With Internet explorer the script works fine. But Firefox have two types of
cache: Memory cache and disk cache.
With firefox disk cache, the script works fine, because don´t appears in the
disk cache, but i have problems with the Memory cache.

How can i avoid to appears in the firefox memory cache?