until now, I only charged via usb on my mac. today I got an AC adapter for fast charging (2.1A) and very strangely, after plugging it in, the kindle just started the normal kindle OS instead of Android 4.0 (with actually the old owner’s account singned in) and won’t go back.
I now set it to my own account, but I would like to go back to the Android 4.0 installation that I’m used to.
Is there an easy way to do this, like a boot loader that let’s you choose between the two, or just changing the default settings for startup somewhere? I don’t know how it was rooted in the first place, but I found an app named "burrito-root" in the applications.
thanks a lot. I didn’t find anything concerning this with google, but I would also be thankful for links to obvious solutions, in case I just missed them 
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Source:
http://mobilehowto.eu/2142/automatic-reset-to-kindle-fire-os-when-charging.html