I think having another browser using SpiderMonkey that isn't Gecko-based (assuming they really did start from scratch on the rest of the browser) will be good for the JS ecosystem as it applies to the web (especially since Chakra is probably going away and almost everything except Firefox and Safari is using V8).
Whether having another HTML/CSS engine will be a good thing remains to be seen. They're going to have to get a lot of market share for it to even matter what they do that isn't copying WebKit, Blink, or Gecko (for example, pretty much nobody on the development side cares about Pale Moon even though they've diverged (a lot in some cases) from Gecko).
Either way, until we see actual code or even just a build, it's hard to say.
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