firmware question

No. you can only install firmwares that are being signed.

 

If that was the case we'd all just be downloading firmware files online and restoring to them any time we want.
Not that easy unfortunately

 

There is a new thing called Prometheus, that claims to allow you to do what you are speaking of, however it is very new, and somewhat complicated in nature. You will have to make sure that you save your "blobs" or "shsh2" etc.... It is also possible that you could lose touchID in the process, depending on the individual circumstances. The creator of this method says it would be difficult for Apple to correct this, and prevent it, however I would not expect it to last too far into the future. I would guess that with IOS11, changes will be made to that process.

https://www.reddit.com/r/jailbreak/comments/5kttrg/tutorial_simple_step_by_step_guide_to_use_the/