I'm sorry for the lack of specific details, my head was just running over.
I'm afraid my first topic overhere will turn out to be rather useless, since the main problem is solved, although I don't know exactly how... I deleted everything I had on my server, starting all over again with WP 1.2.1, and even a fresh new database.
Kubrick 1.2.6 was installed as described in the read-me, in the root of my site. And it works ! The strange listing-problem in Firefox 1.0 is gone !
Little question that remains : How can I remove the topmargin for the header image, so that the page will be "fixed" to the bottem of the adress bar/tabs of the used browser ?
Thanks a lot !