This is sort of like saying "it would be nice to have a concrete proof that P = NP." Determining what's right and wrong is the major endeavor of several entire fields of scholarship. No one can give you the ultimate answer to this question. You can develop your own approximation of the answer by empathizing with other people and reading lots of philosophy, literature, and history.

I think Mozilla is trying to remind techies to think about ethics for this exact reason: a lot of us have this weird mindset where, unless we can define something with mathematical precision, then it doesn't exist and we don't have to worry about it. This is an incredibly unhealthy way to think about ethics, and it's one of the reasons why our field is sliding towards a bad reputation.