I mean, if I want my original firmware back, uninstall it and i rebooted my device, it will display my original display?
Usually uninstalling rockbox is accomplished by reinstalling the OF. Check the manual for a given device if you need to know more.
Can you recommend a firmware for this device?
http://goo.gl/fZUfX
For some generic mp3 player? Probably not. Its off topic here anyway.