I did not find any java related package by using the search string "Mozilla". The problem is that there is no java6-plugin package available for amd64.
I decided to install icedtea-gcjwebplugin package again. Now it's working quiet good!
First. not, probably i did not restart Iceweasel after the first installation.
IcedTea provides a working browser plugin for 64-bit browsers under 64-bit Linux. This makes it suitable to enable support for Java applets in 64-bit Firefox, so also Iceweasel.
For more information, click here