Mozilla building browser-based Unreal 3 engine
Mozilla making the Web a gaming platform with Unreal 3 engine in a browser | Ars Technica
About time. HTML5 was made with things like this in mind, as far as extensibility goes. It was just a matter of time before a large engine project was to be designed. I think Mozilla will do a good job, assuming they aren't stingy and deliberately make it Firefox-only supported (e.g. using their naming conventions instead of cross-browser naming conventions).
But either way, it's about time. Can't wait to see what comes of it.