I think there's some confusion here. According to my understanding installation of Rockbox on the Gigabeat Fx0 involves replacing one of the original firmware files with a bootloader that will boot Rockbox. The OF bootloader never gets touched in this process. So, unless you have done something very unorthodox with your player or my understanding of Rockbox on the Gigabeat is somehow flawed, you are looking for the original firmware, not the bootloader.