Thank you so much! :-)

It also works with fennec instead of Midori in your listing. Maybe this should be placed in the wiki as a Howto?

But there is still one issue left for me:
I have to run
/home/user/browser-proxy.py
one time. Only after this I am able to start fennec using the "normal" Bookmark-Application in the global browser menu (first icon on the left menu).

How I can start the browser.py in the boot process?

EDIT: Seems to be no solution for me. After changing:
/usr/share/dbus-1/services/com.nokia.osso_browser.service

to:

[D-BUS Service]
Name=com.nokia.osso_browser
Exec=/home/user/browser-proxy.py

and restarting my tablet most of the other applications don't start. For example the mail-application and mauku don't start. xterm don't accepts parameter and so on...