It's not only the space bar but some other keys don't work on some of the games either (for example w,a,s,d). In other games they all work fine. Homeland Defense, Midnight Invaders, Earth Rock Hunters, The Matrix: Tunnel Recon, Pang...to name some of my favorites that cause problems. I just installed Firefox to try a different browser. Same problem.
On all other sites in the web that I frequently visit I have no problems using the space bar or the above named keys.