Thanks for the report! This is actually not Firefox-specific -- the copy violation depends on the presence of the "document.referrer" tag to verify that the game is being loaded from a whitelisted domain (and not some other website trying to steal the game). If you have the referrer header disabled, then this obviously won't work.
To change the config value you can go to about:config and change the network.http.sendRefererHeader value back to the default of "2".