Puffs-chan, The firmware is like Windows is to your PC, it is the operating software of your player. You may find a newer link for s1res in the Downloads>Tools section. s1res is a theming tool if you want to change the look of the user interface but you may want to become a little more familiar with your player before trying this.
At this stage I would suggest just backing up your firmware in case of any problems. Get to know your player and it's features, then you may like to make some small modifications to your firmware like changing the battery icon or boot and shutdown animations.
Have a look through the Download>Themes section and you will see some of the themes that members have created for their players.
Most important have fun and enjoy your player
