This article offers the most complete information about iPhone crashing issue. You can find different causes and solutions here, and solve the problem yourself.
Firmware password is also known as EFI. However, if you forget your EFI password then it may be a problem. Here you will learn how to unlock your Mac EFI.
There are three ways of to fix "how to get rid of someone else's Apple ID on my iPhone", when you have a transferred device and you can contact the previous owner.