Error message not shown on disabled cookies

I have a vaadin app (version 6.8.13) and want to show users with disabled cookies a warning message.
This is already part of vaadin for several years.

I debugged the problem and can see that the method
AbstractApplicationServlet.criticalNotification()
is called for a UIDL request.
This method adds some javascript to the response.
But in the Browser (Firefox 27) the error-message does not appear.
I used WebDeveloper Addon and monitored the network, but could not find that message which has been added in the method criticalNotificcation to the response.

Any suggestions on making this work would be appreciated.

Thanks,
Andreas