peppolon,
Upgrading your firmware is very risky as you can quite easily kill your player and it may never work again. There are so many players and different firmware know that it is not possible to say which firmware you can safely use.
Before you do anything to your player you should backup your firmware with s1fwx which you can download from
HERE and there is aguide to using it
HERE.
"Rollyflex" posted information on how he managed to add file browsing to his firmware
HERE.