I get an error on the console 'Permission denied to get property UnnamedClass.classes'
This error occurs when a script does not have enough permission to refer to an XPCOM component. For some reason, Mozilla uses UnnamedClass to refer to the Components object. In effect, the error should be read as "Permission denied to get property Components.classes"
You won't be able to create or call XPCOM components with unprivileged code such as that on a remote site. You will have to sign the code in this case. Chrome installed files should not generate this error.
You might also get this error if the underlying XBL that implements an element tries to call an XPCOM component. You shouldn't encounter this though. However, you cannot call the clipboard or drag and drop convenience scripts in the global directory from a remote site.