I was looking at the list, seems like everyone is opting for Chromium over Gecko. I wonder why? A few go with Webkit.
Apple users are more likely to use whatever Apple sets as the default vs switching web browsers to a rival brand, so even if you beat Apple building from Apple's code base (in terms of quality) you probably still are not going to get much share. Plus what if Apple shifts their code base dramatically in a way that only works on their hardware? They did do away with the Safari for Windows browser years ago.
Chrome has the most marketshare on desktop (bundling with Adobe Flash security updates, heavily marketed across Google properties, etc.) & on mobile (via Android). So if you use Chromium as your base you are getting something that should both work cross-platform & be something many web developers have optimized their code against.