And ... listen to your customers. " Sun Keith had a good point in telling me that version 0.4.1pre2 destroys the header view. Yes, I managed to read all headers without any other extension in 0.4.1pre2 but, ignored existing (already showed) mail headers. Cause the new extension adds headers to the header view and hides them, so where existing header hidden.

Well, this one repaired and the now released pre-version 0.4.1pre3 should solve the problem. Thanks Keith (and all the others) for testing and reporting.