Thanks "hand of fate" for your reply and solution.
Indeed the trick lies in installing (and having your browser use) the nppdf.so plugin. With debian it was a bit different, I had to install the "mozilla-acroread" package which installs nppdf.so and took over the handling of PDF files. These were handled before by mozplugger (mozplugger.so).
So it now works perfectly. The setup can be verified by using the "about

lugins" URL in mozilla/firefox, it should list "Adobe Reader 7.0" (nppdf.so) before any other PDF plugin.
Matter is closed (for me).
PhilippeP