I think that would be because you are not using the newest version of ArcadeOX. I made a change to the site today that might have caused that problem for older versions.
I think I fixed this error so you should be able to play games with out that problem now.
But if you want to upgrade click this link http://arcadeox.com/arcadeox1.6.xpi
and follow the instructions.
There are a few minor improvements between the version you are using and the newest version, but the biggest difference is it is now compatible with Firefox 3.6 so if you want you can upgrade FF too.