Up to now the Unity game engine, which has been very popular with indie game developers, has offered a browser plug-in for those game developers interested in offering their game as a browser-based solution. Now thanks to cooperation with Mozilla and through WebGL, EmScripten, and ASM.js technologies, Unity will now be able to run natively in the browser.
For showcasing the work, Mozilla and Unity are showing the "Dead Trigger 2" game running in the browser at near-native speeds this week during the Game Developers' Conference. When I am at GDC in San Francisco later in the week, I'll check it out!
As talked about recently, Mozilla recently brought Unreal Engine 4 to the browser in conjunction with Epic Games, among other engines and games that Mozilla has been working to bring to the web.
More details on the Mozilla and Unity partnership that was just announced can be found via the Mozilla.org blog.