They used emscripten, which compiles LLVM to Javascript. Demos and info at unrealengine.com.
Mozilla unveils browser-based Unreal Engine 3 using JavaScript | Joystiq
Firefox producer Mozilla teamed up with Epic Games to bring Unreal Engine 3 to the web, without the need for plugins such as Flash. Mozilla and Epic took only four days to port Epic’s engine to the web by using JavaScript code. The results, as demonstrated by this video unveiled at GDC, certainly look impressive.