I'm afraid this announcement will turn out to be disastrous for the open web as we know it. Even if some enthusiasts now will be able to use Firefox proper, the real change is that WebKit usage will more than half over night. After this I guess the rendering engine market share will be roughly 80/20 Chromium/Webkit with Gecko as some rounding error.
For how long will it be defensible for the average company to test their site on anything but Chromium?