I deleted my original comment before you replied because I am not really in the mood to defend this but the OP was talking about the pain of developing for different browsers and I don't care what is a fork of what, this is a fact: Chrome, Firefox and Safari all render differently and have to be catered to individually.
Also, Safari, between desktop and mobile, has 30% of the market to Firefox's 8%.
I don't LIKE it, but there are "effectively" three, not two, rendering engines.