I found the same problem as you did.  ???

http://gwenaelm.free.fr/2008-9/articles.php?lng=fr&pg=1207
It is working for IE7 but it is not working for firefox.

I would suggest you copy ejs_users_ntnu_fkh_magnetdeflect_fr.jar from file directory at http://gwenaelm.free.fr/Physique/Physchim/c03/Mecanique/Html/Gravitation/manipulation__deuxime.html to file directory for http://gwenaelm.free.fr/2008-9/articles.php
and change the applet tag from
[code]
archive="http://gwenaelm.free.fr/Physique/Physchim/c03/Mecanique/Html/Gravitation/ejs_users_ntnu_fkh_magnetdeflect_fr.jar"[/code]
to
[code]archive="ejs_users_ntnu_fkh_magnetdeflect_fr.jar"[/code]
, and find out if it is working.