<parameter key="_moz_dirty"/> in XML Content

Dear Silva developers

I use Silva 2.0.8 and Mozilla Firefox 3.0.6.
Some of my documents contain the following code in the Parsed XML Code:

   <parameter key="_moz_dirty"/>

This leads to the following exception once I edit the document in the 
kupu editor:
   Exception Type: AttributeError
   Exception Value: No field _moz_dirty

Any idea, what might be wrong there?

Thanks in advance
Nico