Please do not waste time using hard to read colors. Have you read the manual and taken the time to read all the prompts from the rockbox utility? It should be pretty clear as the devs have spent a lot of time on documentation. The installation docs provide you a link to sansa and I believe - the rockbox utility also provides information on needed firmware file.
By the way, the firmware does not go on the device.
What problem are you having determining what firmware to use - exactly. I have found sansa to be very thorough with listing their firmware downloads.
good luck