Using firefox-16.0a2 (16 aurora), the demo runs but there are some problems. This is running Ubuntu Maverick on a 8 core system with Radeon 5700 series GPU.
When selecting low-resolution mode, the frame-rate maxes out at about 8. Selecting high-resolution mode the resolution sits at around 16. Both modes are way too slow for this machine, but the low-res mode is even worse.
The character textures do not load. The wall, water, weapon, bullet textures are all there, but the player characters themselves are missing (floating guns).
The sound is has crackling and static. This is even worse in low-res mode.
Do other WebGL demos work ok on your machine? Could be this demo is just too intensive. I don't have much experience with Radeon on Linux though, not sure how powerful that combo is (I tested on NVidia on Linux myself).
Actually, this demo works better than some of the others. Certainly the graphics card is quite powerful and I am using the fglrx driver. So theoretically this should be quite fast. However, it's possible that the age of the driver/distro or just other Linux issue may be affecting performance and correctness.
If those GL drivers are the ones that shipped with Maverick, then they are a bit old. Could be that.