[vladmoz]

As Brendan said, it's UE3, all of it. See the video at http://www.youtube.com/watch?v=BV32Cs_CMqo (second half of video) for the Sanctuary UT3 map, complete with bots, shooting, etc. Once the engine was ported, we threw random UE maps at it, and it worked fine.

It works in any browser that has solid JavaScript and WebGL support. Audio will work with Web Audio, but it'll gracefully not have audio effects if it doesn't. There's no Firefox specific magic here. If you're on Aurora/Nightly, then you get asm.js/Odinmonkey optimizations and it runs even faster; that's it.

Edit: UE3 was chosen because it's known stable and optimized tech. This isn't the pinnacle of what can be done, far from it. But neither us (Mozilla) nor Epic wanted to be working with code that was still under active development for a next generation engine while simultaneously trying to port it to a new experimental platform. One step at a time! :p