Hi,
I don’t know if this helps but I also have an older Raspberry Pi Model B. I successfully installed the 2.1.7 firmware without any problems. I previously had the 2.1.6 firmware installed and was able to do the update from within the 2.1.6 app. In the past I always did a git pull to get the latest update. Have you tried deleting the current firmware you have installed and updating from the repository instead of overwriting the old firmware?
-Doug