I am currently using Java 5. Greenfoot is usually careful to compile everything to Java 5 format class files (even if you develop with Java 6). Dies the project contain classes compiled outside of Greenfoot? If so, you should set a flag on compilation to compile to Java 5 format.
My firefox is using Java version 1.5.0_13 from Apple, but the game applet does not load on this page, but gives this message in the Firefox footer: error: java.lang.UnsupportedClassVersionError: Bad version number in .class file.
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