I don't know if I'm reading this right, but think they aren't competing with "open source" in a general sense, since Webkit is open source. Or maybe you meant

I don't know if I'm reading this right, but think they aren't competing with "open source" in a general sense, since Webkit is open source. Or maybe you meant "with" in the "using" sense: "It's pretty impressive that Apple can compete using open source. I forgot that was possible." Or maybe you were just talking about Firefox...