Okay, thanks much! I've gone to Developer Options and see the button ticked off on Automatic System Updates. I hope that this the only (one) button as that is the only one I see that seems to pertain to System Updates. USB Debugging is also ticked on. Should I be too concerned about using the old firmware in terms of security?
Am I correct in assuming I can download and update freely from the Amazon App Store? I recall that I need to turn off Enable Auto Updates for any Amazon app that shows up under my apps in Google Play. If apps are available on Amazon and Google Play, should I generally go with the Amazon option? In other words is Google Play primarily for the apps not available in the AppStore?
I would not be too concerned with old firmware security unless there are reports of critical security risks.
I don't know about updating Amazon apps, so I can't speak that. But I imagine they should work fine even on outdated OS firmware. One day they may stop working but until that day comes, I am not going to worry about it.