You should answer it just as you said, that you were fired for protecting a customer and not compromising your ethics. Any employer that sees that should understand that statement and ask you for more specific information if they are interested in hiring you.
If a job application asks for a reason for leaving, it is best not to lie. However, a vague answer such as "personal reasons" can be acceptable.
for school leaving certificate
You should answer the question honestly and briefly. There is no reason to go into the details on a job application of your leaving a position.
Educational reasons
A simple, "Thanks and come again."
when customers come back to get more product or service in stead of leaving or complaining.
You could list it as personal reasons.
Looking for a shorter commute.
A person should write a half day leave application to a principal by first explaining why they are leaving. The note should also be signed by a parent.
You need to put on the application that you left job to be at home with your children. No employer would down upon this.
If the large ice masses in the Artic melt, the ocean levels will rise significantly, leaving hundreds of thousands of humans and animals without homes.
An out of station leave application can be something simple like: I am requesting Friday (date) and Monday (date) off. I will be leaving on Friday and returning on Monday, so I will be out of station.